A white gunman was still at large after killing nine people during a prayer service at a historic African-American church in Charleston, South Carolina.
Officers with dogs searched the streets for the suspect, whom police described as a 21-year-old white man with sandy hair wearing a sweatshirt, jeans and boots.
Gunfire erupted inside Emanuel AME Church in downtown Charleston last night, Police Chief Gregory Mullen said.
He described the attack as a hate crime. The gunman had yet to be caught hours and was considered extremely dangerous, he added.
"To have an awful person come in and shoot them is inexplicable, obviously the most intolerable and unbelievable act possible," Charleston Mayor Joe Riley told reporters.
"The only reason someone could walk into a church to shoot people praying is out of hate."
The shooting called to mind a 1963 bombing of an African-American church in Birmingham, Alabama, that killed four girls and galvanised the civil rights movement of the 1960s.
The Charleston church is one of the largest and oldest black congregations in the South, its website says. It has its roots in the early 19th century and the current building, completed in 1891, is considered historically significant, according to the US National Park Service.
The attack follows the April shooting of an unarmed black man in North Charleston by a white police officer. The officer has been charged with murder in that case, one of a number of deaths of unarmed black men in encounters with police that have raised racial tensions in the United States.
Soon after the shooting, a clutch of men stood in a circle in front of a hotel near the church, one of many landmark buildings in the city.
"We pray for the families, they've got a long road ahead of them," Reverend James Johnson, a local civil rights activist, said during the impromptu prayer service.
The FBI, the Bureau of Alcohol, Tobacco, Firearms and Explosives and other agencies have joined in the investigation, Mullen said.
Eight victims were found dead in the church and a ninth person died after being taken to hospital. One other person was wounded and was being treated at a local hospital.
None of the victims were immediately identified. But the Reverend Al Sharpton, the New York-based civil rights leader, said in a tweet that the Reverend Clementa Pinckney, the church's pastor and a member of the state Senate, was among the dead.
After the shooting, a bomb threat was reported near the church, Charleston County Sheriff's Office spokesman Eric Watson said, and people who were gathered in the area were told by police to move back.
Mullen said that the all-clear had been given after checks following the bomb threat.
A police chaplain was present at the scene of the shooting, and a helicopter with a searchlight hovered overhead as officers combed the area.
Following the attack on the church, Republican presidential candidate Jeb Bush, the former governor of Florida, cancelled an appearance in Charleston that had been scheduled for this morning.
"Governor Bush's thoughts and prayers are with the individuals and families affected by this tragedy," his campaign team said in a statement.