Nine people have been shot dead at a historic African-American church in Charleston, South Carolina, and a hunt is under way for awhite gunman.
Police described the attack at the Emanuel African Methodist Episcopal Church as a "hate crime".
They issued surveillance images of the suspect and said he had sat in the church for an hour before opening fire.
The church's pastor, state Senator Clementa Pinckney, is reported to be among the dead.
A prayer meeting was going on at the time of the shooting at about 21:00 local time on Wednesday (01:00 GMT Thursday) at the church in Calhoun Street.
Police described the suspect as white, aged 21-25, clean shaven with a slender build and wearing a grey sweatshirt, blue jeans and Timberland boots.
They released images from surveillance cameras showing him at the church, and also of a black four-door saloon car he was seen driving away in.
Speaking at a news briefing, city police chief Gregory Mullen said that six females and three males were killed, but that names would not be released until all families were notified.
"This is a very dangerous individual who should not be approached," he said.
He said that when police arrived at the scene eight people were already dead in the church and that one other person died later in hospital. There were three survivors, he added.
"This tragedy that we're addressing right now is indescribable," he said. "No-one in this community will ever forget this night."
A woman who survived the shooting told her family that the gunman said he was letting her live so she could report what happened, the Charleston Post and Courier reported.
She said the gunman had sat in the church before standing and opening fire, according to an official from the National Association for the Advancement of Colored People (NAACP).
Charleston Mayor Joe Riley described the shooting as "the most unspeakable" tragedy.
South Carolina Senator Tim Scott tweeted: "My heart is breaking for Charleston and South Carolina tonight."
The campaign of US Republican presidential hopeful Jeb Bush cancelled an event planned in Charleston for Thursday due to the shooting.
"Governor Bush's thoughts and prayers are with the individuals and families affected by this tragedy," his team said in a statement.
Helicopters were seen hovering above the area and a police chaplain was at the scene.
At one point police asked residents to move away because of reports of a bomb - but police later gave the all-clear.
A group of worshippers was seen praying near the church.
"We want some real answers now," one of the worshippers was heard saying.
The attack comes two months after unarmed black man Walter Scott was shot and killed by a white police officer in North Charleston.
The shooting prompted angry protests and highlighted racial tension in the city. The officer has since been charged with murder.
Charleston was also due to hold a ceremony on Thursday marking the eighth anniversary of another tragedy - the death of nine firefighters in a blaze at a furniture store in 2007.