NEW YORK (Reuters) - A gunman who killed two volunteer firefighters and wounded two others in a Christmas Eve ambush in upstate New York left a typewritten note saying he planned to burn down his neighborhood and start "killing people," authorities said on Tuesday.

The gunman, William Spengler, 62, opened fire on volunteer firefighters who responded to a house fire he deliberately set early on Monday morning in Webster, New York, a suburb of Rochester, authorities said.

Spengler shot and killed himself in an ensuing gunfight with police. He had spent 17 years in prison for beating his 92-year-old grandmother to death with a hammer in 1981, authorities said.

Police said Spengler set the fire, laying a trap for the firefighters, and began shooting when they arrived.

He left a typewritten note describing his intent, Webster Police Chief Gerald Pickering said at a televised news conference.

"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," Pickering read from the gunman's statement.

Investigators had found human remains in Spengler's burned-out house, where he lived with his sister, Cheryl Spengler, 67, Pickering said. The remains are assumed to be the sister's and a medical examiner will determine the cause of death.

"A CLEAR AMBUSH"

Spengler started shooting at the arriving firefighters before they got out of their fire truck, Pickering said. He shot out the truck's windshield as they tried to drive away, he said.

"This was a clear ambush on first responders," Pickering said. Police fired back at the gunman, he said. "It was a combat condition. They were shooting at muzzle flash," Pickering said.

Spengler was in a natural depression in an embankment near a tree when he opened fire, he said.

His motive for attacking firefighters remained unknown, Pickering said. It may have been related to contributions his mother had made to the local fire department, he said.

Pickering said he was not sure what the victims were shot with, but said Spengler had an "arsenal of ammunition" and three weapons - a Smith & Wesson .38 caliber revolver, a pump-action Mossberg shotgun, and a .223 Bushmaster rifle with a flash suppressor. Continued...