Investigators believe the man who killed a retired Massachusetts State Police trooper and a U.S. Air Force veteran in Winthrop on Saturday was motivated by hate and acted alone.

Suffolk County District Attorney Rachael Rollins said that David L. Green and 60-year-old Ramona Cooper were “executed” by 28-year-old Nathan Allen. He was later fatally shot by police after he crashed a stolen work truck into a building.

The shootings are being investigated as possible hate crimes because Rollins said “troubling white supremacist rhetoric” in Allen’s handwriting was discovered by investigators. That handwriting contained antisemitic statements and racist statements against Black people, according to Rollins.

Rollins said the only people killed by Allen were Black.

“It’s easy for you to possibly think, ‘This type of thing only happens in Charlotte, or, when we saw the insurrection on Jan. 6, that those were individuals from down south. This happened in Suffolk County,” Rollins said Monday. “This person had some very disturbing beliefs, white supremacist beliefs, regarding… members of our Jewish population as well as Black individuals.”

From the outside and on social media, sources tell 5 Investigates Allen exhibited no warning signs and had no criminal background.

They say there was nothing to indicate Allen was tied to any established hate group. Investigators believe he worked alone.

In response to the attack, officials announced several resources and events being made available to the community. Meredith Hurley, Winthrop’s Director of Public Health, said her office’s crisis intervention team will go door-to-door in the neighborhood and there will be drop-in services offered Wednesday at the Senior Center from 7 p.m. through 8 p.m.

A vigil is also scheduled for 7 p.m. Thursday at Winthrop Town Hall.

Winthrop police Chief Terence Delehanty said his department received a call shortly before 2:45 p.m. Saturday about the crash involving the stolen truck that leveled a building. He confirmed that no one was inside the building at the time of the crash.

After the crash, authorities said Allen exited the vehicle and proceeded to get away. At some point shortly thereafter, Allen is believed to have shot Cooper and Green. Green was pronounced dead at the scene, while Cooper was transported to Massachusetts General Hospital in Boston, where she was later pronounced dead.

Delehanty said one victim, later identified as Cooper, was found by Winthrop police on Shirley Street, about a half-block away from the scene of the crash. Green, meanwhile, was engaged by the suspect in an alleyway between two houses that was further down the street.

According to Rollins, Cooper was shot three times in the back and Green was shot four times in the head and three times in his torso.

Police then engaged Allen, who Delehanty said was armed with two weapons, and at least one officer, a Winthrop police sergeant, shot him at the intersection of Shirley Street and Veterans Road. Allen was transported to Mass General, where he died of his injuries.

At this time, investigators believe Allen acted alone in the crash and shootings. Rollins said Allen was “not on my radar” and that he had a lawful license to carry that transferred to another town, meaning he had nothing on his background check.

In a statement, MSP Superintendent Col. Christopher Mason said Green was murdered outside of his Beach Road home.

According to Mason, Green had an honorable 36-year career in law enforcement and retired on Dec. 31, 2016. He became a Metropolitan District Commission police officer in 1980 and became a state trooper 12 years later when the MDC police were merged into the MSP.

Green spent much of his MSP career assigned to the Boston State Police Barracks at Leverett Circle, Mason said.

Rollins said law enforcement stood around Green’s body at the scene of the shooting. Police officers, firefighters and emergency medical technicians lined Albany Street in Boston as Green’s body was being transported to the Office of the Chief Medical Examiner.

People also left flowers at the site of the crash and the spot where Green was fatally shot.

“Trooper David Green more than upheld the ideals of integrity, professionalism, and service to others that are the hallmarks of a great Trooper. We are heartbroken by his loss and offer our condolences to his family and friends,” Mason said in his statement.

Nick Tsiotos, a longtime friend of Green’s, says he had coffee with him Saturday morning and that the retired trooper, who was also a military veteran, was sitting with friends when he heard the crash and decided to help.

“He thought there was a crash and did not have his weapon with him, obviously,” Tsiotos said. “He went out and tried to do what he was doing for 36 years with the state police: trying to help save lives and help save people.”

Witnesses say once Green realized Allen was armed, he tried to stop the gunman from hurting his friends and neighbors, ultimately losing his life in the process.

“I really believe he saved people’s lives because this gunman was deranged,” Tsiotos said. “Dave probably stopped him from going into homes and killing people.

“There was no better human being than Dave Green,” Tsiotos added. “He really fulfilled everything that was good, the best of humanity.”

Cooper’s son said she was a mother, grandmother and sister, adding, “My mother was a good person. She would help anyone who needed it. She was caring and selfless.”

Rollins said Cooper rose to the rank of staff sergeant in the Air Force and that she was still involved in the military.

One of Cooper’s neighbors, who is also a military veteran, tells NewsCenter 5 that Cooper was a Veterans Affairs employee.

“This is a sad day,” Rollins said Sunday. “These two people protected our rights. They fought for us to be safe and to have the opinions that we have, and they were executed. We will find out why and find out more about this man that did this.”

Another vehicle was involved in the crash with the stolen truck. The two occupants of that vehicle were transported to an area hospital with non-life-threatening injuries.

According to Rollins, Allen was traveling at a rate twice the speed limit in the stolen truck and was heading in the direction of Jewish temples.

“We don’t know where he was going. That is mere speculation,” Rollins said Sunday. “We do know that he had antisemitic rhetoric written in his own hand.”

Sources tell 5 Investigates Allen went out Friday night and never came home.

Allen’s license to carry a weapon was renewed in Winthrop last year, according to 5 Investigates’ sources.

A Winthrop police sergeant who shot Allen was taken to an area hospital to be evaluated, but Delehanty said the officer was not seriously injured.

“It was extremely heroic. He’s a great police officer. He’s a great sergeant. He isolated a significant threat to this community and ended that threat,” Delehanty said Sunday.

“It’s something that we don’t wake up in the morning wishing to do,” the police chief added. “Again, (no officer) was physically harmed, but we are emotionally harmed by our actions that are necessary to protect the community. We want to make sure he’s taken care of today, tomorrow and in the future.”

The sergeant will be placed on administrative leave while Rollins’ office investigates the shooting, per standard procedure. Rollins did say that investigators believe the officer who shot Allen told the suspect to put his gun down multiple times.

The Winthrop Police Department, Massachusetts State Police and Suffolk County District Attorney’s Office are investigating the shooting.