A convicted criminal shot dead two firefighters and wounded two others after luring them to a blaze that police said he had apparently set as a trap in a small town in New York state.
The gunman was named as William Spengler (62). He was found guilty of manslaughter and jailed for 17 years for his grandmother's death in 1980.
After shooting the men he shot himself in the head, police said. He had three guns.
Police also revealed that they have found human remains in the burned-out ruins and believe the victim is the gunman's sister.
Police Chief Gerald Pickering said the remains were found in the charred house that the killer shared with his 67-year-old sister, Cheryl. A medical examiner will need to determine the identity.
Police say William Spengler armed himself with a revolver, shotgun and military-style rifle before he set his house on fire to lure first responders into a pre-dawn death trap on Christmas Eve.
And detectives also revealed how he left a chilling note outlining a plan to: "do what I like doing best, killing people".
Police said the gunman shot at the arriving firefighters, probably with a rifle from on top of an earthen berm. The victims were named as police lieutenant Michael Chiapperini (43), the Webster Police Department's public information officer; and 19-year-old Tomasz Kaczowka, also an emergency dispatcher.
"It does appear it was a trap," town police chief Gerald Pickering said. "These people get up in the middle of the night to go put out fires. They don't expect to be shot and killed."
"We as the community of New York mourn their loss as now two more families must spend the holidays without their loved ones," said governor of New York Andrew Cuomo. Mr Spengler had served more than 17 years in prison for beating his 92-year-old grandmother to death in 1980 at the house next to where the attack happened, Mr Pickering said. Spengler was paroled in 1998 and had led a quiet life since, authorities said.
He lived with his sister, and mother, Arline, who died in October. A friend said Spengler didn't seem violent but "couldn't stand" the sister who lived with him. Roger Vercruysse said Spengler "loved his mama to death". He said he thinks Spengler "went crazy" after she died in October. Two men died at the scene, and two were taken to hospital. A fifth man who was passing by was also injured. After the gunman fired, one of the wounded men managed to flee.
Colleagues described Mr Chiapperini as a "lifetime firefighter" with nearly 20 years with the department, and called Mr Kaczowka a "tremendous young man".
The two wounded firefighters, Joseph Hofstetter and Theodore Scardino, were both awake and expected to recover, authorities said. The incident, which comes as debate rages in the US about gun control following the Newtown school massacre earlier this month, happened shortly before 6am local time in a small lakefront residential community.