The gunman who ambushed two firefighters as they approached his burning house left a note saying he wanted to torch the neighbourhood and "do what I like doing best, killing people".
Gerald Pickering, police chief in Webster, New York state, said 62-year-old William Spengler armed himself with three weapons and set his house on fire to lure emergency services into a death trap.
Spengler killed two volunteer firefighters - Mike Chiapperini, 43, and 19-year-old Tomasz Kaczowka - and shot two others before taking his own life in the small town on Monday morning.
Police believe human remains found in the burnt-out home are those of the gunman's 67-year-old sister, Cheryl, who has been missing since the blaze. The remains will be examined before identity is confirmed.
The full contents of the two-page, typed note - which police said does not reveal a motive - have not been published, but Mr Pickering said it clearly shows the killer's "intent".
One of the guns used was a semi-automatic Bushmaster rifle, the same type of gun used in the December 14 school massacre in Newtown, Connecticut. Mr Pickering said Spengler was "equipped to go to war".
Mr Chiapperini's full-time job was as a public information officer with the Webster police department. He was named firefighter of the year for 2012.
Mr Kaczowka worked as an emergency call dispatcher and Mr Pickering described him as "a tremendous young man".
The two injured volunteer firefighters, Joseph Hofstetter and Theodore Scardino, remain in hospital. They are said to be awake and alert and are expected to recover despite suffering "significant" injuries.
Spengler was released from prison in 1998 after serving 17 years for beating his 92-year-old grandmother to death with a hammer in 1980.
Authorities have said he lived a quiet life and had done nothing to bring himself to police attention since his parole.
Spengler's mother Arline, who had lived with her two children, died in October, leaving Spengler on his own with his sister whom he "couldn't stand", according to neighbour Roger Vercruysse.
Spengler "stayed on one side of the house and [Cheryl] stayed on the other," Mr Vercruysse said.
Dozens of people turned out on Monday night for an impromptu vigil to mourn the firefighters, leaving bouquets and messages of condolence at the local fire station.
One man left a crate of beer, the local newspaper told Sky News.