Four people were killed and another four wounded when a man armed with a rifle opened fire in a restaurant and fled in the nude.
Police are using a helicopter in the hunt for the suspect, who they say is considered “still armed and extremely dangerous”.
Three of those injured are in a critical condition in hospital, and one is critical but stable, ABC News reported.
The gunman fired on diners at a branch of Waffle House near Nashville, Tennessee, in the early hours, after breaking in wearing only a jacket.
Police initially said three people had been killed, and later updated the toll after a fourth victim died in hospital.
Detectives identified the suspect as Travis Reinking, 29, of Morton, Illinois, after finding the vehicle he allegedly used to drive to the restaurant was registered in that name.
A Waffle House customer saw the killer struggling to reload his AR-15-style assault rifle and managed to wrestle away the weapon from him, according to Nashville police, who issued a photo of it.
They said the gunman shed his coat after firing, and fled naked on foot.
They later added that a man believed to be Reinking had been spotted nearby “wearing black pants and no shirt”, and were trying to track him down using a helicopter.
The shooting happened at 3.25am at the Waffle House in the suburb of Antioch, about 16 miles south-east of Nashville.
According to The Tennessean, police believe the death toll could have been much higher if a "hero" had not intervened to grab the rifle.
Chuck Cordero, a witness, said: "He really saved some people. I'm positive he did. Had that guy had a chance to reload his weapon, there was plenty more people in that restaurant."
Authorities are appealing to anyone with information on Reinking to contact them.
In all, 35 officers from three precincts responded to the shooting, arriving to find two of the dead outside and one inside, police said.
The shooting will inevitably revive America’s charged debate about gun laws.
Nashville’s acting mayor David Briley said: “It’s a tragic day for our city anytime people lose their lives at the hands of a gunman.
“My heart goes out to the families and friends of every person who was killed or wounded in this morning’s shooting. I know all of their lives will be for ever changed by this devastating crime.
“There’s clearly more to be said about these circumstances, but for now I ask Nashville to pray for and rally around these victims, and join me in thanking the Metro Nashville Police Department as it works to find and apprehend the shooter.”
Pat Warner, head of public relations for Waffle House, called it a “very troubling” situation.
“We are sending our corporate team from Atlanta and heading to Nashville now,” Mr Warner told ABC. “Our thoughts are with those affected.”
