A masked white man has shot dead three Black people in a shop in Florida, in the southern United States, in a racially-motivated attack, according to local authorities.

According to the sheriff’s office in Jacksonville, the man, who was in his 20s but has not yet been named, was wearing body armour when he entered the Dollar General outlet armed with an assault-style rifle and a handgun.

He shot dead three Black customers – two men and one woman. After a standoff with police, he shot himself.

“This shooting was racially motivated and he hated Black people,” Jacksonville Sheriff T K Waters told a press conference on Saturday.

Waters said authorities believed the attacker acted alone and that before the attack, he had authored “several manifestos” for media, his parents and law enforcement detailing his “disgusting ideology of hate”.

Waters said the rifle had hand-drawn swastikas on it.

He said the attacker was spotted at a local historically Black college, Edward Waters University, where he put on his vest and a mask before going to the local branch of Dollar General, a discount chain with stores across the US.

The victims of the attack have not been named.

Sherri Onks, the special agent in charge of the Jacksonville FBI office, said federal officials had opened a civil rights investigation and would pursue the incident as a hate crime.

“Hate crimes are always and will always remain a top priority for the FBI because they are not only an attack on a victim, they’re also meant to threaten and intimidate an entire community,” Onks said.

Florida Governor Ron DeSantis described the shooting as “horrific” and called the gunman a “scumbag.”

“He was targeting people based on their race, that is totally unacceptable,” said DeSantis, who is running for the Republican nomination for the 2024 presidential race.

There has been a series of shootings in the US in recent days, with easy access to firearms in most states and more guns in the country than citizens.

Earlier on Saturday, at least seven people were taken to hospital after a mass shooting at a Caribbean festival in the northeastern city of Boston, police said.

The night before, two women were shot at a baseball game in Chicago, while a 16-year-old was shot dead and four others hurt after an argument at a high school football game in Oklahoma.

The Jacksonville shooting comes after a self-declared white supremacist killed 10 Black people in a livestreamed shooting rampage at a supermarket in the US state of New York in May 2022.

Payton Gendron planned the attack for months, targeting Tops Friendly Market in Buffalo because of the large African American population in the surrounding neighbourhood. He pleaded guilty to the killings in November.