Police say the human remains found inside the home that William Spengler set ablaze in Webster, N.Y. Monday belonged to his 67-year-old sister who lived in the house with him.
Officials say Spengler, 62, set his house on fire and then proceeded to shoot at firefighters that attempted to put it out.
Volunteer firefighters Mike Chiapperini and Tomasz Kaczowka were shot and killed while responding to the blaze.
Two other first responders are in the hospital and are expected to recover.
Police say Spengler used three different weapons to attack those on the scene. One of them was a Bushmaster semiautomatic rifle, the same type of rifle used in the Newtown, Conn. school massacre.
Spengler was later found dead -- killed by a self-inflicted gunshot to the head.
Police found a typewritten note from Spengler.
"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," the note read.
No motive was provided in the letter.
He was a convicted felon who served 17 years for manslaughter in connection with the death of his grandmother in 1980.
Felons cannot legally possess weapons.
That attack happened in the home next door to where Monday's incident took place.
"We're assuming he was shooting from a high-ground advantage at a berm near the home, where he had barricaded or set himself up with his weapons to shoot the innocent responders," Webster Police Chief Gerald Pickering said.
The shooting forced fire crews to evacuate the area and leave the scene, leaving the fire to burn.
It eventually destroyed seven homes and a car.