A white man shot dead three Black people in a racially motivated attack in a Florida discount store Saturday before taking his own life after a standoff with police, authorities 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," Jacksonville Sheriff TK Waters told a news conference about the gunman, who was in his early 20s.

According to the sheriff's office, the shooter, who has not yet been identified, entered a Dollar General store wearing a tactical vest, armed with an AR-style rifle and a handgun.

Manifestos discovered by the gunman's family shortly before the attack "detail the shooter's disgusting ideology of hate," Waters said, and at least one of the guns had hand-drawn swastikas on it.

The shooting took place near Edward Waters University, a historically Black college in the southern U.S. state.

The university said in a statement that the shooter had been on campus earlier that day, though no one was harmed.

"An on-campus Edward Waters University security officer engaged an unidentified male in the vicinity of the Centennial Library on campus," it said.

"The individual refused to identify themselves and was asked to leave."

The university added that the individual – later identified as the shooter – left "without incident."

The FBI will investigate the shooting as a hate crime, said Sherri Onks, the bureau's special agent for Jacksonville, a city of nearly 1 million in the northeast corner of the state.

There was no evidence the shooter was part of a larger group, officials said.

"We know that he acted completely alone," Waters said.