A man who shot dead three people in a racially motivated attack in Florida wrote of his hatred of Black people, police have said.

Twenty-one year old Ryan Christopher Palmeter fired eleven rounds at one woman sitting in her car in Jacksonville, before entering a shop and shooting another two people.

Sheriff T K Waters said he then turned the gun on himself.

Mayor Donna Deegan said it was a "hate-filled crime" driven by racist hatred.

At a news conference on Sunday, Mr Waters confirmed Palmeter had no previous criminal history and lived with his parents in Clay County.

Palmeter had authored several manifestos, for his parents, the media and federal agents, detailing his hatred of Black people, police said.

Mr Waters said those manifestos "detailed the shooters disgusting ideology of hate".

"Finely put: this shooting was racially motivated and he hated black people."

"The manifesto is, quite frankly... the diary of a madman", he said. "He knew what he was doing. He was 100% lucid. He knew what he was doing and again, it's disappointing that anyone would go to these lengths to hurt someone else".

Palmeter acquired his weapons legally, police said

Mr Waters said Palmeter had been briefly detained for 72 hours in 2017 under the Baker Act, mental health legislation that allows the involuntary detainment of an individual for treatment.

But the sheriff said his weapons had been acquired legally, telling reporters the problem was not with the availability of guns, but with the killer being "a bad guy".

He urged people not to "look for sense in a senseless act of violence".

Jacksonville police played CCTV video at the news conference showing the moment Palmeter walked up to the car where he killed the first woman with his gun loaded. It then cut to video of him entering the shop.

Mr Waters also confirmed that Palmeter let some people out of the shop without injuring them.Media caption,

Watch: Jacksonville shootings: 'He hated black people'

"Why? I don't know. Some of them were white, but I do believe there was a couple that were not," he said.

US Attorney General Merrick Garland said on Sunday the Justice Department was "investigating this attack as a hate crime and an act of racially motivated violent extremism".

"No person in this country should have to live in fear of hate-fuelled violence and no family should have to grieve the loss of a loved one to bigotry and hate," he said.

The attack happened less than a mile from the historically black Edwards Waters University.

Palmeter first went to the university campus, where he was asked to identify himself by a security officer, the university said in a statement. When he refused, he was asked to leave.

"The individual returned to their car and left campus without incident," the statement added.

Sheriff Waters said the gunman was then seen putting on a bullet-resistant vest and a mask before leaving the campus.

The university went into lockdown after the shooting.