At least 10 people were killed on Saturday in a shooting at a supermarket in Buffalo, New York, law enforcement officials told The Associated Press.
A US attorney has said the FBI is investigating the shooting as both a hate crime and racially motivated violent extremism.
It comes as the mayor of Buffalo mayor said the suspect “travelled hours” to supermarket where 13 were shot, 10 fatally.
Details on the number of additional people shot at the Tops Friendly Market and their conditions were not immediately available.
Buffalo police said earlier on Saturday afternoon that the alleged shooter, who was not identified, was in custody.
The two officials were not permitted to speak publicly on the matter and did so on the condition of anonymity.
Police officials and a spokesperson for the supermarket chain did not immediately respond to messages from The Associated Press seeking comment.
The gunman entered the supermarket with a rifle and opened fire, one of the officials said. Investigators believe the man may have been live-streaming the shooting and are looking into whether he had posted a manifesto online, the official said.
The official cautioned the investigation was in its preliminary stages and that authorities had not yet discerned a clear motive, but are investigating whether the shooting was racially motivated.
The supermarket is in a predominately black neighbourhood, about three miles north of central Buffalo.
Witnesses reported the gunman was wearing military-style clothing and body armour, one of the officials said.
Braedyn Kephart and Shane Hill, both 20, pulled into the car park just as the shooter was exiting. They described him as a white male in his late teens or early twenties sporting full camo, a black helmet and what appeared to be a rifle.
“He was standing there with the gun to his chin. We were like what the heck is going on? Why does this kid have a gun to his face?” Mr Kephart said. He dropped to his knees. “He ripped off his helmet, dropped his gun, and was tackled by the police.”
Police closed off the block, lined by spectators, and yellow police tape surrounded the car park.
Mayor Byron Brown and Erie County Executive Mark Poloncarz were at the scene late on Saturday afternoon, gathered in a car park across the street from the Tops store and expected to address the media.
Governor Kathy Hochul tweeted that she was “closely monitoring the shooting at a grocery store in Buffalo”, her hometown. She said state officials have offered help to local authorities.
The Erie County Sheriff’s Office said on social media that it ordered all available personnel to assist Buffalo police.