William Spengler, who allegedly have lured two volunteer firefighters to their deaths in Webster, New York, left a long note outlining his plans, police say.
They told a news conference that the gunman, 62-year-old William Spengler, had three guns and aimed to burn down his neighborhood.
No motive was given but the note said he was planning to “do what I like doing best – killing people.”
The firefighters were shot dead as they arrived to tackle a fire on Monday.
Seven homes on a narrow stretch of land beside Lake Ontario were destroyed by the fire. Another two firemen and an off-duty police officer were wounded in the attack.
Police Chief Gerald Pickering told reporters that Spengler, who apparently shot himself in the head, had used a .38 calibre revolver, a 12-gauge shotgun and a Bushmaster .223 calibre rifle with flash suppression.
The same make and calibre rifle had been used in the murder of 26 teachers and children at an elementary school in Newtown, Connecticut on 14 December, he said.
The police chief told reporters in Webster that a two- to three-page note from the gunman had been found in which he said: “I still have to get ready to see how much of the neighborhood I can burn down and do what I like doing best – killing people.”
William Spengler had served 17 years in jail for killing his grandmother with a hammer but had done nothing to attract the authorities’ attention since being granted parole in 1998.
As a convicted criminal, he was not allowed to own weapons.
Police said he appeared to have set “a trap” by setting fire to his home. His older sister, Cheryl, whom he reportedly hated, is missing.
Two of the firefighters who responded to the call, Tomasz Kaczowka, 19, and Mike Chiapperini, aged 43, were shot dead in what was described as a “clear ambush” on emergency services.
Their two wounded colleagues are said to be in a stable condition with gunshot wounds.
The off-duty police officer, who was also shot and wounded as he came to their aid, was praised by the police chief for saving people’s lives.
Gerald Pickering cited “mental health issues” as a possible factor for the attack, which came months after the death of William Spengler’s mother.
The Newtown killer, Adam Lanza, killed his mother with her own guns before launching his attack on Sandy Hook elementary school in Connecticut.
His murders of 20 children aged six and seven have prompted a national debate on gun ownership and mental health.
“It’s sad to see that that this is becoming more commonplace in communities across the nation,” Monroe County Sheriff Patrick O’Flynn told AP news agency after the attack in New York state.