A gunman driven by racial hate shot and killed three Black people in Jacksonville, Florida on Saturday before fatally turning the gun on himself, authorities said.

The shooter, a white male in his early 20s, was outfitted with a tactical vest and armed with an AR-style rifle and a handgun when he started firing inside a Dollar General discount store, Sheriff TK Waters said.

"He targeted a certain group of people and that's Black people. That's what he said he wanted to kill. And that's very clear," the sheriff said, adding there were two male victims and one female victim.

Manifestos discovered by the gunman's family shortly before the attack "detail the shooter's disgusting ideology of hate," Waters told a news conference.

The FBI would investigate the shooting as a hate crime, said Sherri Onks, the bureau's special agent for Jacksonville.

The shooting was the latest in a spate of gun violence this weekend in the United States, following deadly outbursts in Boston, Chicago and Oklahoma.