The call to the police department in Bristol, a town in central Connecticut about 80 miles northeast of New York City, came at 10:29 p.m. on Wednesday, reporting a possible domestic incident between two siblings, Sgt. Christine Jeltema of the Connecticut State Police said.

When the officers arrived, the suspect, Nicholas Brutcher, was standing outside the home on Redstone Hill Road and immediately began firing, officials said. Mr. Hamzy was shot dead at the scene, while Sergeant Demonte was taken to the hospital and pronounced dead there.

Mr. Brutcher, 35, was shot and killed outside the home, the State Police said. His brother, Nathan Brutcher, 32, was also shot and taken to a nearby hospital for treatment. His condition is unknown.

Norberto Rodriguez, who lives across the street from where the shooting occurred, said he saw a man dressed in camouflage emerge from the house with a gun. Another man ran out, apparently to restrain him, and the man with the rifle shot him, Mr. Rodriguez said.