New York (CNN) -- A man who ambushed firefighters Monday, fatally shooting two of them as they arrived to put out a raging inferno in upstate New York, took his own life with gunshot to the head, according to an autopsy conducted by the Monroe County Medical Examiner's Office.
Two other volunteer firefighters were wounded in the attack in the Rochester-area town of Webster.
Authorities do not know how William Spengler, 62, who was found dead hours after the four firefighters were shot, obtained the weapon or weapons he used or why he opened fire.
As a convicted felon, Spengler could not legally possess guns, but he had "several different types of weapons," Webster Police Chief Gerald Pickering said. Spengler was convicted in 1981 of first-degree manslaughter in the death of his grandmother and had been released on supervised parole.
"It appears that it was a trap," Pickering said. "There was a car and a house that were involved in flames, probably set by Mr. Spengler, who laid in wait in armament and then shot the first responders."
In audio heard over a scanner, a West Webster Fire Department firefighter reported "multiple firemen shot" -- including himself, with wounds to his lower back and lower leg -- and "shots still being fired."