The white gunman who shot and killed three Black people at a Dollar General store in Jacksonville, Florida was a 21-year-old who bought his guns legally and had no criminal history.

The shooter, Ryan Christopher Palmeter, lived with his parents in a suburb of Jacksonville and had died of a self-inflicted gunshot wound, Sheriff T.K. Waters told a news conference on Sunday.

Waters has said the shooting was racially motivated. Authorities said the shooter left behind several manifestos for media, his parents and law enforcement detailing his hatred for Black people.

“There was no criminal record, nothing,” Waters said, adding the only thing on file was a domestic violence call with his brother.

“There were no red flags.”

However, the sheriff said Palmeter in 2017 was briefly held under a state law called the Baker Act, which states a person can be “taken to a receiving facility for involuntary examination” during a mental health crisis.

Reuters could not immediately reach members of the gunman’s family for comment.

The gunman also left a will and a suicide note that were retrieved by his father, the  sheriff said.

President Joe Biden in a statement on Sunday noted the shooting occurred the same day the country marked the 60th anniversary of the March on Washington, scene of Martin Luther King Jr’s famous “I have a dream” speech.

“We must refuse to live in a country where Black families going to the store or Black students going to school live in fear of being gunned down because of the colour of their skin,” Biden said.

The Justice Department is investigating the shooting as a hate crime and an “act of racially motivated violent extremism,” Attorney General Merrick Garland said in a statement. Garland said the shooting would be investigated as a hate crime.

Waters said the suspect was caught on video shooting Angela Michelle Carr, a 52-year-old woman, in her car outside the Dollar General, a US discount chain. He then entered the store where he shot and killed 19-year-old Anolt Joseph “AJ” Laguerre Jr and Jerrald De’Shaun Gallion, 29.

Palmeter wore a tactical vest and his face was covered by a mask, Walters said. He carried a handgun and an “AR-15 style” rifle, with swastikas on it, police said, referring to a lightweight semi-automatic long gun often used in mass shootings.

Authorities said he was spotted trying to enter a local historically Black college, Edward Waters University, on Saturday before heading to the Dollar General store.

Palmeter had refused to identify himself to a campus officer and was turned away.

“It was later determined that the individual would be involved in a shooting near EWU campus,” the university said.

Florida Governor Ron DeSantis said on Sunday the state would work with the school to ensure it has adequate security following Saturday’s shooting.

DeSantis, a Republican presidential candidate, has been sharply criticised by Black leaders for what they say is his government’s attack on Black history.

In January, Florida banned an Advanced Placement course on African-American history from being offered to high school students. In July, the state changed how history is taught in schools.

DeSantis was booed when he began to speak at a prayer vigil for the three victims on Sunday.

Jacksonville city councilwoman Ju’Coby Pittman, a Democrat who represents the area where the shooting took place, chastised the crowd and told them the governor was there to help.

DeSantis then told the crowd the gunman was “a major-league scumbag” and said in Florida “we’re not going to let people be targeted based on their race”.

Shortly after DeSantis spoke, Jeffrey Rumlin, a pastor at the Dayspring Church in Jacksonville, took the microphone and said of the shooter: “At the end of the day, respectfully, governor, he was not a scumbag. He was a racist.”

—AAP