A gunman killed three people in a racially motivated attack and then killed himself in Jacksonville, Florida, the city's Sheriff said.
The man, described as white and in his early 20s, entered a Dollar General store and opened fire, triggering a standoff with police, BBC reported.
Sheriff T.K. Waters said on Saturday that two men and a woman were killed by the gunman, who wore body armour and left manifestos.
Mayor Donna Deegan said it was a "hate-filled crime" driven by racist hatred.
The Sheriff said the shooter -- who has not yet been officially named -- carried a lightweight semi-automatic rifle and a handgun, BBC reported.
He is believed to have acted alone and allegedly wanted to kill himself. He lived in Jacksonville's Clay County with his parents and "authored several manifestos", Sheriff Waters said, including one to his parents and another to the media.
The FBI has opened a civil rights investigation into the shooting, which it is treating as a hate crime.
Jacksonville Mayor Donna Deegan told local TV channel WJXT: "One shooting is too much but these mass shootings are really hard to take."
In a statement provided to the BBC's US partner, CBS News, Dollar General said it was "heartbroken by the senseless act of violence that occurred at our Kings Road store", adding that "supporting our Jacksonville employees and the DG family impacted by this tragedy is a top priority as we work closely with law enforcement".
There have been more than 28,000 gun deaths in the US so far this year, according to the Gun Violence Archive website.
The Jacksonville attack comes on the 60th anniversary of the March on Washington and Martin Luther King Jr's famous "I have a dream" speech.