The mass shooting took place at the Tops Markets on Jefferson Avenue. The gunman was an 18-year-old white male, said Buffalo Police Commissioner Joseph Gramaglia at a news conference.
He is under custody.
Gramaglia said 11 of the shooting victims were Black, while two others were white.
FBI is investigating the shooting as a hate crime and as an act of racially-motivated violent extremism.
The shooter was "very heavily" armed and was wearing tactical gear and a tactical helmet, according to Gramaglia.
"He had a camera that was live streaming what he was doing," he added.
Senator Chuck Schumer of New York, who is also Senate Majority leader, condemned the shooting and offered sympathy for the victim's families.
"I ache for the victims and their families. I ache for Buffalo. I ache for the tight-knit East Side community. Racism has no place in our state or our country. I’ve spoken with the Mayor of Buffalo and offered any help we can provide. We stand with the people of Buffalo," he wrote on Twitter.
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