A convicted felon shot dead two firefighters and wounded two others Monday after luring them to a blaze that police said he had apparently set as a trap in a small town in New York state.
The 62-year-old shooter - who was found guilty of manslaughter and jailed for 17 years for his grandmother's death in 1980 - then fatally shot himself in the head, said police in Webster, a suburb of Rochester on the shores of Lake Ontario.
His sister, who lived with him, was nowhere to be found, according to police. Investigators had not yet determined the gunman's motive.
The incident, which comes as debate rages in the United States about gun control following the Newtown school massacre earlier this month, happened shortly before 6:00 am (1100 GMT) in a small lakefront residential community.
"It does appear that it was a trap that was set for our first responders," Webster police chief Gerald Pickering told a press conference, his voice breaking with emotion.
"People who get up in the middle of the night to fight fires, they don't expect to get shot and killed," he added.
"We are a safe community, a tragedy like this is just horrendous."
The firefighters were shot as they approached the scene of the blaze - a car and a house engulfed in a fire that they now believe was set intentionally by the shooter, identified as William Spengler.
"Four of the firefighters were shot. Two are deceased, two were transported to area hospitals," Pickering said.
The two injured men were not in critical condition and were both expected to recover.
Another off-duty police officer who responded was also shot and wounded.
One of the injured firefighters - all volunteers in the small suburb of Rochester - was able to escape and call for help.
"We are being shot at. Multiple firemen down. Multiple firemen are shot. I am shot. I think he is using an assault rifle. We have multiple firemen down. Working fire," said the injured man, according to a police scanner recording broadcast by Fox News.
A security cordon was put up around the scene and residents were evacuated. Seven homes were destroyed in the blaze, as the shooting thwarted initial efforts to douse the flames.
Pickering said emergency personnel had not yet been able to search the torched homes for possible additional victims.
New York Governor Andrew Cuomo said state police and the Office of Emergency Management were working with local law enforcement to respond to the scene.
He also offered his condolences to the families of the victims of the "horrific shooting" and "senseless act of violence."
"New York's first responders are true heroes as they time and again selflessly rush toward danger in order to keep our families and communities safe," Cuomo said in a statement.
"We as the community of New York mourn their loss as now two more families must spend the holidays without their loved ones."
The incident in Webster came 10 days after a shooting rampage at an elementary school in Newtown, Connecticut that saw a disturbed young man gun down 20 children, aged six and seven, and six adults.
The shooter, Adam Lanza, had killed his mother at their home before heading to the school, where he eventually took his own life.
The Newtown shooting has revived debate in the United States on the country's gun laws, which are far more lax than in most other developed nations.
President Barack Obama said he would support a new bill to ban assault rifles, and has put Vice President Joe Biden in charge of a panel looking at a wide range of other measures, from school security to mental health.
Democratic Senator Dianne Feinstein has pledged to introduce a bill in January that would ban at least 100 military-style semi-automatic assault weapons, and would curb the transfer, importation and possession of such arms.
But the nation's most powerful gun lobby, the National Rifle Association, strongly opposes any new restraints in gun sales, with the group's executive vice president Wayne LaPierre calling Feinstein's proposal "phony."
The United States has suffered an explosion of gun violence over the last three decades, including 62 mass shooting incidents since 1982.