A sniper who ambushed volunteer firefighters in upstate New York on Monday, killing two and seriously wounding two others, left a note saying he hoped to burn down his neighborhood and kill as many people as possible, police said Tuesday.

A charred body, believed to be his sister's, was found in the burned house she shared with him Tuesday, police said.

William Spengler, 62, used a Bushmaster semiautomatic rifle, the same kind of weapon used in the assault on Sandy Hook Elementary School, Webster Police Chief Gerald Pickering said.

"He was equipped to go to war," Chief Pickering said.

The shooter, who was convicted of killing his grandmother decades ago, was found dead of a self-inflicted gunshot wound hours later.

Pickering, at a news conference Tuesday, read a sentence from the three-page typewritten note that detectives believe Spengler's left behind: "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 indicated Spengler's intentions, but not his motive, Pickering said. The rest of the contents will not be made public because it is evidence in a criminal investigation, he said.

There is "all kinds of speculation" about why Spengler wanted to destroy his neighborhood and kill firefighters and residents, Pickering said.

One theory is that he was upset about a donation his mother, who died in the past year, made to the fire department, he said. Another theory is there could be a connection to his arrest in the killing of his grandmother, he said.

"Motive is always the burning question and I'm not sure we'll ever really know what was going through his mind," Pickering said.

Spengler was convicted in 1981 of first-degree manslaughter in the death of his grandmother and had been released on supervised parole, Pickering said.

It will be a challenge for the medical examiner to determine if his siste r-- 67-year-old Cheryl Spengler was killed before the fire was set because it was a "raging inferno," Pickering said.

Spengler's former neighbor, Roger Vercruysse, said that Spengler was a nice guy who used to come over to Vercruysse's sister's house for holiday parties and would wave to the family from his front porch, where he often sat during the summer.

"He'd come to our house, we used to have picnics," he said.

Spengler was especially attentive to his mother, who passed away in October, Vercruysse said, visiting her every day in the nursing home where she lived until she died.

"He loved his mama," Vercruysse said. "He always talked about his mother."

Spengler did not share the same closeness with his sister, with whom he shared his home, Vercruysse said.

"He told me he hated his sister and never could tell me why," he said. "I'd always wave to the sister, but she was not friendly."

Firefighters from the Rochester-area town of Webster responded before 6 a.m. Monday to a 911 call, reporting a fire that Spengler is believed to have set, when the gunfire began, Pickering said.

"This was a clear ambush on first responders," he said. Spengler was firing from "a natural depression" against a bank and a tree, he said.