(CNN) -- 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 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 he 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 William Spengler's sister -- 67-year-old Cheryl Spengler -- was killed before the fire was set, because it was a "raging inferno," Pickering said.
A 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.
An off-duty police officer, who happened on the scene, returned gunfire and sheltered firefighters with his car, Pickering said.
"Had the police officer not been there, more people would have been killed because he immediately engaged the shooter with a rifle," he said. "Essentially, it was a combat condition." Investigators won't know until after an autopsy if any of his shots hit Spengler, he said.
Officer John Ritter of the Greece, New York, Police Department, suffered minor shrapnel wounds but was released after treatment at a hospital.
The two wounded firefighters were in stable condition after surgery Tuesday, Pickering said Tuesday morning. They were being treated for "serious injuries" in intensive care at Strong Memorial Hospital in Rochester, New York, he said.
Seven houses were destroyed and several others damaged by the fire, which investigators believe spread from a car parked next to the home where they believe Spengler lived, Pickering said.
Authorities do not know how Spengler obtained the Bushmaster rifle, .38-caliber revolver and 12-gauge shotgun he used, Pickering said. As a convicted felon, Spengler was not allowed to legally possess weapons.
In chilling audio heard over a scanner Monday, 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."
"I'm pretty sure that we have two DOAs" -- the term for dead on arrival -- "on the street," the wounded firefighter said. "... They're down and not good."
For several hours after that, the threat of gunfire stopped firefighters from battling the blaze and forced police SWAT teams to evacuate 33 people in the neighborhood of small waterfront homes.
The fire destroyed seven houses. It was under control by 2:30 p.m. ET, but authorities weren't able to get into any of the homes. Pickering said it's possible that more victims could be inside.
Lt. Michael Chiapperini, a firefighter who died at the scene, was a veteran of the West Webster Fire Department and a police lieutenant. He'd been named Firefighter of the Year just two weeks ago. And not long before that, he had volunteered to go to Long Island to help those suffering after Superstorm Sandy, New York Lt. Gov. Bob Duffy said.
The other slain firefighter was Tomasz Kaczowka, who was also a 911 dispatcher. He'd been with the West Webster Fire Department for just more than a year, department spokesman Al Sienkiewicz said.
The shooting occurred amid a renewed gun control debate after the December 14 elementary school massacre in Newtown, Connecticut, that killed 26 people, most of them children. The gunman in that case, Adam Lanza, also killed his mother and himself.
The head of a lobbying group that represents first responders said the Monday shooting was "senseless and cruel."
"The firefighters who responded today were performing a selfless, meaningful service to their community, unaware that a cold-hearted maniac was planning to ambush them and take their lives," said Harold Schaitberger, general president of the Washington-based International Association of Fire Fighters. "Coming on the heels of the horrific tragedy in Newtown, Connecticut, and on Christmas Eve, this shooting is even harder to comprehend."
New York Gov. Andrew Cuomo described the Webster shooting as "horrific." And the state's attorney general called it a "senseless tragedy"
President Barack Obama has set a January deadline for "concrete proposals" to deal with gun violence after the Newtown shooting.
Sen. Dianne Feinstein, D-California, has said she will introduce legislation to reinstate the assault weapons ban that expired in 2004, while National Rifle Association CEO Wayne LaPierre has said his group will fight any new gun restrictions, saying most gun laws now on the books are rarely enforced.
Pickering, the Webster police chief, said it was important -- after the shooting in his town and others -- to "get a handle on gun control." He also said more needs to be done to make sure that dangerous people aren't in society, where they can kill.
"For the last 20 years, we have been turning people loose and de-institutionalizing people, and I think we've swung too far," he said. "I think there are still people that need to be in institutions that are a danger to themselves or others. And this is a classic example."
CNN's Brittany Brady, Chuck Johnston, Jake Carpenter and John Fricke contributed to this report.