Investigators found human remains in the burned-out home of William Spengler, 62, a day after his rampage in Webster, a Rochester suburb. Officials said the remains probably were those of Spengler's missing sister, Cheryl, 67.
Spengler apparently set the fire in or near his home and lay in wait, killing two firefighters and seriously wounding two more before killing himself, officials said.
In his note, Spengler wrote: "I still have to get ready to see how much of the neighborhood I can burn down and do what I do best: killing people," police said at a Christmas Day news conference.
Officers said the note did not include a motive. They declined to release more excerpts.
"Motive is always the burning question, and I'm not sure we'll ever really know what was going through his mind," Webster Police Chief Gerald Pickering said at a news conference.
Spengler appeared to have led an uneventful life for the last 14 years, but had a violent history. In 1980, he beat his grandmother to death with a hammer and spent 18 years in prison. He was released in 1998.
Monday's fire - which officials think may have started as a vehicle fire - consumed seven homes and damaged two more in Webster, a community on the shores of Lake Ontario and Irondequoit Bay.
Police think Spengler used a military-style Bushmaster .223-caliber rifle with a flash suppressor in his rampage. They recovered the weapon along with a Smith and Wesson .38-caliber revolver and a Mossberg pump-action 12-gauge shotgun.
As a felon, Spengler was barred from owning guns. Officials were unsure how he had obtained them, but said he was armed to the teeth.
"He was equipped to go to war and kill innocent people," Pickering said.
Spengler's attack was the third in two weeks in which a gunman has killed multiple victims with with an assault rifle.
On December 14, Adam Lanza killed 20 grade-school students and six adults at Sandy Hook Elementary School in Newtown, Conn., using a military-style Bushmaster .223-caliber rifle. Lanza also killed his mother and himself. On December 11, Jacob Tyler Roberts opened fire in a Clackamas, Oregon, mall with an AR-15-style rifle, killing two and wounding one before taking his own life.
The discovery that Spengler had a Bushmaster is likely to intensify the outcry for tighter gun controls that followed the Newtown massacre. The gun, which looks like a military assault rifle, also was used in the 2002 sniper attacks that left 10 dead and three wounded in the Washington, D.C., area.
President Barack Obama is expected to begin pushing for specific gun control measures, including bans on assault weapons and high-capacity magazines, after he receives recommendations from Vice President Joe Biden in January. The National Rifle Association, the nation's most powerful gun lobby, has said it would fight any attempts to curtail gun ownership.
In Webster, during the first of two news conferences Tuesday, officials described Monday's chaotic "combat situation." Firefighters were targeted before getting out of their trucks, police said, and a Webster police officer used his duty rifle to trade fire with Spengler.
Rounds shattered the windshield of the firetruck that two of the firefighters were in; the wounded driver crashed trying to get away.
"Had that police officer not been there, more people would have been killed, because he immediately engaged the shooter," Pickering said of the officer, who has not been identified.
A police officer from nearby Greece, N.Y., Jon Ritter, was driving behind the firetruck when he also came under fire. He was wounded by shrapnel from the bullets that struck his windshield and engine block, police said.
Ritter "tried to shelter some of the fallen firemen with his car when the other firefighters-that we later extracted from the location with the armored personnel carrier-had taken cover under the firetruck to try to escape further harm from the ongoing gunfire," Pickering said.
The two wounded firefighters remained in intensive care.
Officials said 33 neighborhood residents had been displaced by the blaze and the investigation and that hotels had offered them places to stay.
"We all have been inundated from citizens, police agencies across the nation and really across the world, wanting to provide donations," Pickering said. The outpouring "has been incredible."