The US government will seek the death penalty for a white man accused of killing nine black parishioners at a historic church in Charleston, South Carolina, last June.
"The nature of the alleged crime and the resulting harm compelled this decision," Attorney General Loretta Lynch said in a statement on Tuesday.
Dylann Roof, 22, is accused of opening fire on June 17, 2015 during a Bible study session at Emanuel African Methodist Episcopal Church. The killings shook the country and intensified the debate about race in America.
He faces 33 federal charges, including hate crimes, obstruction of religion and firearms offences. Authorities have accused him of holding white supremacist views, saying he targeted the victims because of their race.
His federal trial had been delayed while US prosecutors decided whether to seek the death penalty. Defence lawyers have said he would plead guilty if he did not face the possibility of execution.
He also faces the state death penalty if convicted of the shooting.
Roof's lawyer, Michael O'Connell, declined to comment on the prosecution's decision when reached by phone on Tuesday.
The government said in court documents it would prove a number of factors that would justify a death sentence, saying Roof had planned the killing and showed a lack of remorse.
Joseph Meek, Roof's 21-year-old friend who pleaded guilty last month to federal charges related to the shooting, said Roof had planned the shooting for six months and wanted to start a race war.
Federal prosecutors rarely seek the death penalty against defendants. Since the federal death penalty was reinstated in 1988, about three people a year on average have been sentenced to death, according to the Death Penalty Information Center, a nonprofit group in Washington.
The most recent death penalty carried out by the federal government occurred in 2003, when Louis Jones was executed by lethal injection for the kidnapping and murder of a 19-year-old Army private.
Boston Marathon bomber Dzhokhar Tsarnaev was sentenced to death in May 2015 for helping carry out the 2013 attack that killed three people and wounded 264 others at the race's finish line.