A convicted felon who committed suicide after shooting dead two firefighters he lured into an inferno wrote in a note that killing people was his favorite pastime, police said on Tuesday.
Authorities also said they found human remains in the charred skeleton of his house that most likely belonged to the gunman's sister, who lived with him.
"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," read part of the note discovered in the wake of Monday's shooting in the upstate New York town of Webster, police chief Gerald Pickering told reporters.
He said the typewritten note by 62-year-old William Spengler was two or three pages long, but would not be released at this time.
Investigators suspect the victim found in what was left in the house is Cheryl Spengler, 67. She had not been seen since Monday. The brother and sister were said to not get along.
Pickering was unable to confirm whether Cheryl Spengler had died in the fire or if she was killed beforehand. A medical examiner is working on identifying the remains.
After setting his home and car ablaze, Spengler took up a sniper position, waiting for firefighters to arrive. He was armed with three weapons and an "arsenal" of ammunition, according to Pickering.
He had a Smith & Wesson .38 caliber revolver, a Mossberg 12-gauge pump action shotgun and a .223 Bushmaster rifle- the same military-type assault rifle used in the recent elementary school shooting in Connecticut that left 26 people dead.
As soon as the firefighters arrived on the scene, Spengler shot at them. His rampage came to a halt thanks to a policeman who returned fire.
"This was a clear ambush on first responders," Pickering said. "Essentially, it was a combat mission."
In addition to the two firefighters killed, two of their colleagues sustained "serious injuries" and underwent surgery, Pickering said, noting they were in stable condition. A police officer was treated and has been released from hospital.
Seven homes were destroyed in the blaze, as the shooting thwarted initial efforts to douse the flames.
Even before the shootout, Spengler had already been found guilty of manslaughter and jailed for 17 years for his grandmother's death in 1980.
The incident in Webster came 10 days after a shooting rampage at an elementary school in Newtown, Connecticut, that saw a disturbed young man gun down 20 children, aged six and seven, and six adults.
The shooter, Adam Lanza, had killed his mother at their home before heading to the school, where he eventually took his own life.
The Newtown shooting has revived debate in the United States on the country's gun laws, which are far more lax than in most other developed nations.