Buffalo police confirmed the shooter was in custody in a tweet, but they did not identify the suspect.
Details on the number of additional people shot on Saturday at the Tops Friendly Market and their conditions were not immediately available.
“Ten people were killed by a gunman dressed in body armour and armed with a high-powered rifle, while three others were wounded – two of them critically,” The Buffalo News reported citing a police official at the scene and another source close to law enforcement.
Investigators believe the man may have been live-streaming the shooting and were looking into whether he had posted a manifesto online, a police official told the Associated Press.
The official cautioned that the investigation was in its preliminary stages and that authorities had not yet discerned a clear motive, but were investigating whether the shooting was racially motivated.
The supermarket is in a predominately Black neighbourhood, about 3 miles (5 kms) north of downtown Buffalo. The surrounding area is primarily residential.
Buffalo police confirmed the shooter was in custody in a tweet, but did not identify the suspect. Police officials and a spokesperson for the supermarket chain did not immediately respond to messages seeking comment.
Braedyn Kephart and Shane Hill, both 20, pulled into the parking lot just as the shooter was exiting. They described him as a white male in his late teens or early twenties sporting full camouflage, 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?” Kephart said. He dropped to his knees. “He ripped off his helmet, dropped his gun, and was tackled by the police.”
Mayor Byron Brown and Erie County Executive Mark Poloncarz were at the scene late Saturday afternoon, gathered in a parking lot 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.
The Erie County Sheriff’s Office said on social media that it ordered all available personnel to assist Buffalo police.
The shooting comes little more than a year after a March 2021 attack at a King Soopers grocery in Boulder, Colorado, that killed 10 people.