The gunman, William Spengler, 62, opened fire on volunteer fire-fighters who responded to a house fire he deliberately set early on Monday morning in Webster, New York, a suburb of Rochester, authorities said.
Spengler shot and killed himself in an ensuing gunfight with police. He had spent 17 years in prison for beating his 92-year-old grandmother to death with a hammer in 1981, authorities said.
Police said Spengler set the fire, laying a trap for the fire-fighters, and began shooting when they arrived.
He left a typewritten note describing his intent, Webster Police Chief Gerald Pickering said at a televised news conference.
"I still have to get ready to see how much of the neighbourhood I can burn down and do what I like doing best, killing people," Pickering read from the gunman's statement.
Investigators had found human remains in Spengler's burned-out house, where he lived with his sister, Cheryl Spengler, 67, Pickering said. The remains are assumed to be the sister's and a medical examiner will determine the cause of death.
"A CLEAR AMBUSH"
Spengler started shooting at the arriving fire-fighters before they got out of their fire truck, Pickering said. He shot out the truck's windshield as they tried to drive away, he said.
"This was a clear ambush on first responders," Pickering said. Police fired back at the gunman, he said. "It was a combat condition. They were shooting at muzzle flash," Pickering said.
Spengler was in a natural depression in an embankment near a tree when he opened fire, he said.
His motive for attacking fire-fighters remained unknown, Pickering said. It may have been related to contributions his mother had made to the local fire department, he said.
Pickering said he was not sure what the victims were shot with, but said Spengler had an "arsenal of ammunition" and three weapons - a Smith & Wesson .38 calibre revolver, a pump-action Mossberg shotgun, and a .223 Bushmaster rifle with a flash suppressor.
The same model Bushmaster rifle was used in the killing of 20 students and six teachers in Newtown, Connecticut, on December 14.
Authorities with the Bureau of Alcohol, Tobacco, Firearms and Explosives were checking on the origin of the weapons, he said.
As a convicted felon, Spengler could not legally own guns. Pickering said authorities were examining potential links between the weapons and thefts of guns in the county.
The fire-fighters killed in the attack were Lieutenant Michael Chiapperini and Tomasz Kaczowka, authorities said.
The injured fire-fighters were identified as Joseph Hofsetter and Theodore Scardino. Both men were listed in guarded condition at Strong Memorial Hospital in Rochester.
An off-duty police officer was wounded by shrapnel as he drove past the scene. The officer was treated at a hospital and released.
Seven homes in the neighbourhood were destroyed by the fire, and two were uninhabitable, Pickering said.