Olha Havryliuk’s son and son-in-law, along with a stranger, were shot in the head in the yard of their house. The Russian soldiers smashed the Havryliuks’ fence, parked their armored vehicle in the garden, and moved into the house. They cooked in the neighbor’s garden, killing and plucking chickens and roasting them on a barbecue while the men lay dead yards away across the alley.

By the time the troops pulled out at the end of March, two brothers, Yuriy and Viktor Pavlenko, who lived at the end of the street, lay dead in a ditch by the railway line. Volodymyr Cherednychenko was found dead in a neighbor’s cellar. Another man, caught by the Russian soldiers as he ran along the train track and taken into a cellar of a house at the end of the street, was also found shot dead.

The story of Bucha and its horrors has unfolded in chapters as new revelations of Russian atrocities emerge, fueling outrage among Ukrainians and across much of the world. But prosecutors and military intelligence officials were investigating early on, collecting evidence to try to identify the perpetrators responsible for the mass killings, torture and rapes in the once tranquil suburb.

Working with war crimes and forensic experts from around the world, Ukrainian investigators have reached some preliminary conclusions, focusing in particular on the 64th Brigade. They have already identified 10 soldiers from the unit and accused them of war crimes.

Ukrainian officials say that the brigade was formed after Russia struggled in a 2008 war with Georgia, and that it was awarded an honorary title by President Vladimir V. Putin last month for its performance in Ukraine.

Yet the brigade took little part in any fighting, coming in after other units had seized control of Bucha and then tasked with “holding” it. The troops established checkpoints throughout the town, parking their armored vehicles in people’s yards and taking over their homes.

“They imprisoned our people,” said Ruslan Kravchenko, the chief prosecutor for the Bucha district, describing the actions of the accused soldiers. “They tied their hands and legs and taped their eyes. They beat them with fists and feet, and with gun butts in the chest, and imitated executions.”

The name of the 64th Brigade and a list of 1,600 of its soldiers were found among computer files left behind in the Russian military headquarters in Bucha, providing investigators with an immense resource as they began their investigation. Dmytro Replianchuk at Slidtsvo.info, a Ukrainian investigative news agency, soon found the social media profiles of dozens of the names, including officers.

Three victims who survived beatings and torture have been able to identify the perpetrators from the photographs, Mr. Kravchenko said.

One of the victims was Yuriy, 50, a factory worker, who lives near one of the most notorious Russian bases, at 144 Yablunska Street. On March 13, a unit of the 64th Brigade came to search his house. He said that he had identified the soldiers when shown photographs by prosecutors. The soldiers were rough and uncouth, he said. “You could see they were from the Taiga,” he said, referring to the Siberian forest. “They just talk to bears.”

Yuriy managed to avoid suspicion, but on March 19, the soldiers returned and detained his neighbor Oleksiy. Like several others interviewed for this article, the men asked to be identified by only their first names for their security.

Oleksiy declined to be interviewed but confirmed that he had been detained twice by the Russian unit, interrogated in a basement for several hours and put through a mock execution when the soldiers fired a gun behind him. Still shaken, he said, “I just want to try to forget it all.”

Created to ‘Scare the Population’

Based in Russia’s far east, near the border with China, the 64th Brigade belongs to the Eastern Military District, long seen as the part of the Russian Army with the lowest levels of training and equipment.

The brigade has ethnic Russian commanders but consists largely of soldiers drawn from minority ethnic groups and disadvantaged communities, according to Col. Mykola Krasny, the head of public affairs of Ukrainian military intelligence.

In radio conversations that were intercepted by Ukrainian forces, some of the Russians expressed surprise that village roads in outlying areas of Kyiv, Ukraine’s capital, were paved with asphalt, he said.

“We see it as a deliberate policy to draft soldiers from depressed regions of Russia,” Colonel Krasny said.

Not a lot is known about the brigade, but Colonel Krasny claimed that it was notable for its lack of morality, for beatings of soldiers and for thieving. Drawn from a regiment that had served in Chechnya, the brigade was established on Jan. 1, 2009, shortly after Russia’s war in Georgia, Colonel Krasny said. The goal was clear, he added: to build up a fearsome army unit that could instill control.

“The consequences of these politics was what happened in Bucha,” he said. “Having no discipline, and these aggressive habits, it looks like it was created to scare the population.”

He claimed that the Russian soldiers’ disadvantaged backgrounds, and the fact that they could act with impunity, prompted them “to do unspeakable things.”

It was not only the enemy who suffered their brutality. The Russian Army has long had a reputation for hazing its own soldiers, and on a cellphone left behind in Bucha by a member of the 64th, investigators found recent evidence of the practice: a video in which an officer is talking to a subordinate and then suddenly punches him in the side of the head while other soldiers stand around talking.

The Russian government did not respond to a request for comment on the accusations against the 64th Brigade but has repeatedly claimed that allegations of its forces having committed atrocities in Bucha and elsewhere are false.

Western analysts who have studied the Russian Army said that the behavior of troops in Bucha was not a surprise.

“It is consistent with the way they consider responding,” said Nick Reynolds, a researcher of land warfare at the Royal United Services Institute, a military research organization in London. “Reprisals are part and parcel of how the Russian military does business.”

The ‘Bad Guys’ Will Come

Killings occurred in Bucha from the first days that Russian troops appeared. The first units were airborne assault troops, paratroopers and special forces who fired on cars and civilians in the streets and detained men suspected of being in the Ukrainian Army or territorial defense.

The extent of the killings, and the seeming lack of hesitation among Russian soldiers to carry them out, has led Ukrainian officials to surmise that they were acting under orders.

“They couldn’t not know,” Bucha’s prosecutor, Mr. Kravchenko, said of senior military commanders. “I think the terror was planned.”

Many of the documented killings occurred on Yablunska Street, where bodies lay for weeks, visible on satellite images. But not far away, on a corner of Ivana Franka Street, a particular form of hell played out after March 12.

Residents had already been warned that things would get worse. A pensioner, Mykola, 67, said that the Russian troops who first came to the neighborhood had advised him to leave while he could. “‘After us, such bad guys will come,’” the commander told him, he recalled. “I think they had radio contact and they knew who was coming, and they had their own opinion of them.”

Mykola left Bucha before the 64th Brigade arrived.

The spring flowers are pushing up everywhere in Bucha, fruit trees are in blossom, and city workers have swept the streets and filled in some of the bomb craters. But at the end of Ivana Franka Street, amid smashed cars and destroyed homes, there is an eerie desolation.

“From this house to the end, no one is left alive,” said Ms. Havryliuk, 65. “Eleven people were killed here. Only we stayed alive.”

Her son and son-in-law had stayed behind to look after the house and the dogs, and were killed on March 12 or 13, when the 64th Brigade first arrived, she said. The death certificates said that they had been shot in the head.

What happened over the next two weeks is hard to fathom. The few residents who stayed were confined to their homes and only occasionally dared to go out to fetch water from a well. Some of them saw people being detained by the Russians.

Nadezhda Cherednychenko, 50, pleaded with the soldiers to let her son go. He was being held in the yard of a house and his arm had been injured when she last saw him. She found him dead in the cellar of the same house three weeks later, after the Russians withdrew.

“They should be punished,” she said of his captors. “They brought so much pain to people. Mothers without children, fathers, children without parents. It’s something you cannot forgive.”

Neighbors who lived next door to the Havryliuks just disappeared. Volodymyr and Tetiana Shypilo, a teacher, and their son Andriy, 39, lived in one part of the house, and Oleh Yarmolenko, 47, lived alone in the other side. “They were all our relatives,” Ms. Havryliuk said.

Down a side alley lived Lidiya Sydorenko, 62, and her husband Serhiy, 65. Their daughter, Tetiana Naumova, said that she spoke to them by telephone midmorning on March 22.

“Mother was crying the whole time,” Ms. Naumova said. “She was usually an optimist, but I think she had a bad feeling.”

Minutes later, Russian soldiers came in and demanded to search their garage. They told a neighbor to leave, shooting at the ground by her feet.

“By lunchtime they had killed them,” Ms. Naumova said.

She returned to the house with her husband, Vitaliy, and her son Anton last month after the Russian troops withdrew from Kyiv. Her parents were nowhere to be found, but they found ominous traces — her father’s hat with bullet holes in it, three pools of blood and a piece of her mother’s scalp and hair.

There was also no sign of the Shypilos or of Mr. Yarmolenko, except trails of blood where bodies had been dragged along the floor of their house.

Eventually, French forensic investigators solved the mystery.

They examined six charred bodies found in an empty lot up the street and confirmed that they were the missing civilians: the Sydorenkos, the three Shypilos and Mr. Yarmolenko. Several bore bullet wounds but three of them had had limbs severed, including Ms. Naumova’s mother, the investigators told the families.

Her father had multiple gunshot wounds to the head and chest, her mother had had an arm and a leg cut off, she said.

“They tortured them,” Ms. Havryliuk said, “and burned them to cover their tracks.”

Oleksandr Chubko contributed reporting from Bucha, Ukraine.

The Texas shooting, which left 19 schoolchildren and two teachers dead and more than a dozen wounded, has put a national spotlight on Daniel Defense, a family-owned business in Georgia that has emerged as a trailblazer in an aggressive, boundary-pushing style of weapons marketing and sales.

Some of its advertisements invoke popular video games like “Call of Duty” and feature “Star Wars” characters and Santa Claus, messages that are likely to appeal to teenagers. The company was an early adopter of a direct-to-consumer business model that aimed to make buying military gear as simple as ordering from Amazon, enticing customers with “adventure now, pay later” installment plans that make expensive weaponry more affordable.

And the company’s founder and chief executive, Marty Daniel, has fashioned himself as a provocateur who ridicules gun control proposals and uses publicity stunts to drum up sales.

Daniel Defense is at the forefront of an industry that has grown increasingly aggressive in recent years as it tries to expand beyond its aging, mostly white customer base and resists the calls for stronger regulation that seem to intensify after every mass shooting.

“Daniel Defense is basically the poster child of this egregious, aggressive marketing,” said Ryan Busse, a former executive at the gun company Kimber who is now an industry critic. “Marty Daniel burst in the door, a lot louder and more brazen than other gun makers, much like Donald Trump did on the political scene.”

He added, “Through this company, you are telling the story of how the gun industry has become increasingly radicalized.”

Daniel Defense’s strategy seems to have been effective. Its sales have soared, in part because of its successful targeting of young customers like Salvador Ramos, the gunman in Texas. Mr. Ramos, whom the authorities killed on Tuesday, was a “Call of Duty” video game enthusiast and appears to have bought his assault rifle directly from Daniel Defense, less than a week after turning 18.

Mr. Daniel did not respond to emails or calls. Steve Reed, a Daniel Defense spokesman, said in a statement that the company was “deeply saddened” by the Texas shooting.

Mr. Daniel, 59, is a practiced storyteller who adopts a folksy tone to market his company and its guns. He often casts himself as something of a goofball, a screw-up who flunked out of Georgia Southern University — not once, but twice — before finally graduating and starting a company that made garage doors.

He has said that his gun company was born out of his poor golf game. Instead of puttering around the course, Mr. Daniel started using an AR-15 — the type of gun he would later go on to make — for target practice. “Every shot he fired filled him with a satisfaction he’d never before experienced,” the company’s website says.

At the time, Mr. Daniel had trouble finding a way to mount a scope onto his rifle. He began designing and selling his own accessory that allowed gun owners to add lights, a range finder and lasers onto the rifle.

He got his break in 2002 at a gun show in Orlando, Fla., where he was approached by a representative of the U.S. Special Forces. He ultimately won a $20 million contract to produce the accessories for combat rifles. More deals followed. In 2008, he won a contract with the British military, according to Daniel Defense’s website.

By 2009, the company had expanded to making guns for consumers. Its military ties were the basis of its marketing, which often featured heavily armed fighters. “Use what they use,” one ad says. Another shows a military-style scope aimed at passing cars on what looks like a regular city street. Others include references — using hashtags and catchphrases — to the “Call of Duty” video game.

Before the 2000s, most gun makers did not market military-style assault weapons to civilians. At the largest industry trade shows, tactical military gear and guns were cordoned off, away from the general public. That started to change around 2004, industry experts say, with the expiration of the federal assault weapon ban.

“Companies like Daniel Defense glorify violence and war in their marketing to consumers,” said Nick Suplina, a senior vice president at Everytown for Gun Safety, a group that supports gun control.

In 2012, the Sandy Hook shooting led to an industrywide surge in gun sales, as firearm enthusiasts stocked up, fearing a government crackdown. In an interview with Forbes, Mr. Daniel said the shooting “drove a lot of sales.” (Forbes reported that Daniel Defense had sales of $73 million in 2016.)

After the shooting, Daniel Defense offered employees extra overtime to meet skyrocketing demand, according to Christopher Powell, who worked for the company at the time. “They kept people focused on the task at hand,” he said.

But in the late 2010s, some colleagues started to worry that Mr. Daniel had become distracted by the glamour of marketing the brand and rubbing shoulders with celebrities and politicians, according to a former Daniel Defense manager. They voiced concerns that some of the marketing materials were inappropriate for a company that manufactures deadly weapons, said the manager and a former executive, who didn’t want their names used because they feared legal or professional repercussions.

Some ads featured children carrying and firing guns. In another, posted on Instagram two days after Christmas last year, a man dressed as Santa Claus and wearing a military helmet is smoking a cigar and holding a Daniel Defense rifle. “After a long weekend, Santa is enjoying MK18 Monday,” the caption states, referring to the gun’s model.

The industry’s aggressive marketing has landed some companies in trouble. Earlier this year, the gun maker Remington reached a $73 million settlement with families of children killed at the Sandy Hook school in Newtown, Conn. The families had claimed that Remington improperly marketed its assault rifles, including with its weapons appearing in “Call of Duty,” which the killer at Sandy Hook had frequently played.

A year after Sandy Hook, with the Super Bowl approaching, Daniel Defense deployed a new marketing stunt.

The National Football League had a policy prohibiting ads for weapons on its telecasts. But Daniel Defense tried to buy a 60-second spot that depicted a soldier returning home to his family, with ominous music in the background. “I am responsible for their protection,” the ad’s narrator intones. “And no one has the right to tell me how to defend them.”

Given the N.F.L.’s ban on gun ads, it was no surprise that the ad was rejected. (Daniel Defense claimed that the ad complied with the policy because the company sells products besides guns.) But Mr. Daniel turned the rejection into a rallying cry, and the conservative media lapped it up. Appearing on Fox News’s “Fox & Friends,” he urged viewers to “call the N.F.L. and say, ‘C’mon, man, run my ad.’”

“That is Marty Daniel at work,” Mr. Powell said. “He’s not one of those typical C.E.O.s that you see.”

Mr. Daniel and his wife, Cindy, have worked hand-in-hand with the National Rifle Association to raise money for the group, sell weapons to its members and beat back calls for gun control.

In recent years, Mr. Daniel and Ms. Daniel, the company’s chief operating officer, became outspoken supporters of Donald J. Trump, contributing $300,000 to a group aligned with Mr. Trump. Mr. Daniel joined the “Second Amendment Coalition,” a group of gun industry heavyweights who advised Mr. Trump on gun policy.

Mr. Daniel told Breitbart News in 2017 that Mr. Trump’s election saved “our Second Amendment rights.” He and his wife have also donated to other Republican candidates and groups, including in their home state of Georgia. So far in the 2022 election cycle, they’ve given more than $70,000 to Republicans.

Before the Uvalde massacre, Daniel Defense’s guns were used in at least one other mass shooting. Four of its semiautomatic rifles were found in the hotel room of the gunman who killed 59 people at a Las Vegas music festival in 2017, one of the deadliest shootings in American history.

Mr. Daniel has been an especially vocal critic of gun control. After the shooting at Parkland High School in 2018, he briefly expressed support for legislation, backed by the N.R.A., to bolster the federal background-check system. But he soon reversed his position, citing “overwhelming feedback.” He declared that “all firearms laws that limit the rights of law-abiding citizens are unconstitutional.”

“You don’t see the same kind of boldness from the chief executives of Smith & Wesson or the old-guard gun companies,” said Josh Sugarmann, executive director of the nonprofit Violence Policy Center. “Daniel is more at the edges.”

Daniel Defense is only a fraction of the size of those rivals. It manufactured nearly 53,000 guns in 2020, the most recent year for which government data is available, giving it a less than 1 percent share of the market.

But experts say it has led the way in building a direct-to-consumer sales business, as gun manufacturers try to match the success of other industries in capitalizing on e-commerce.

In the past, gun companies would sell their products to stores, which then sold the weapons to customers. Now, industry experts say, the manufacturers are increasingly trying to sell guns and accessories online, targeting consumers with slick ad campaigns. (Guns sold online have to be picked up at a licensed firearms dealer, who conducts a background check.)

Daniel Defense also offers a buy-now-pay-later financing option that allows qualified buyers to spread the price — some of its guns retail for more than $1,800 — over a number of payments. The approval takes seconds, the company’s website says.

“They’ve been a brand leader,” said Timothy Lytton, a law professor at Georgia State who studies the gun industry. “They’ve been exceptionally successful at selling the idea that civilians who’d like to own a firearm for self-protection need a high-capacity, semiautomatic weapon.”

Gun sales surged during the pandemic, including at Daniel Defense. The company also received help via a $3.1 million loan from the federal Paycheck Protection Program, which was intended for small businesses at risk of laying off employees.

The week before the Texas shooting, Daniel Defense posted a photograph on Facebook and Twitter, showing a little boy sitting cross-legged, an assault rifle balanced across his lap. “Train up a child in the way he should go,” the caption reads, echoing a biblical proverb. “When he is old, he will not depart from it.”

The ad was posted on May 16. It was Mr. Ramos’s 18th birthday.

A day later, he bought his first gun, a Smith & Wesson assault-style rifle, from a store in Uvalde, according to State Senator Roland Gutierrez of Texas who cited law enforcement officials. The store has been identified as Oasis Outback. Three days later, he bought the Daniel Defense rifle for $1,870 plus tax, according to a photo of the receipt that Mr. Ramos reportedly posted on the social media platform Yubo.

Amid a national outcry after the shooting, Daniel Defense retreated from its usual provocative online presence. The company restricted access to its Twitter feed. It canceled plans to have a booth at this weekend’s N.R.A. convention in Houston.

And on Thursday, it removed the $15,000 guns-and-ammo sweepstakes from its home page.

Tara Siegel Bernard and Serge F. Kovaleski contributed reporting and Kitty Bennett contributed research.