Details on the number of people shot at the Tops Friendly Market and their conditions weren't immediately available. Buffalo police said 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 AP 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 were 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 hadn't yet discerned a clear motive, but were investigating whether the shooting was racially motivated.
The supermarket is in a predominantely black neighbourhood, about five kilometres north of downtown Buffalo. The surrounding area is primarily residential, with a Family Dollar store and fire station near the store.
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 parking lot 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?" 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 taped surrounded the full parking lot. 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. 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.
The shooting came little more than a year after a March 2021 attack at a King Soopers grocery in Boulder, Colorado, that killed 10 people. Investigators have not released any information about why they believe the man charged in that attack targeted the supermarket.
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