The suspect Dylann Roof is awaiting trial.
The white gunman accused of killing nine black parishioners at a Charleston, South Carolina, church last year was slightly injured in jail by another inmate early on Thursday, Charleston County Sheriff Al Cannon said on Thursday.
Dylann Roof, 22, was outside his cell and getting ready to shower at the North Charleston jail where he is detained when the assault occurred, the sheriff said.
Cannon said he did not know what motivated the 25-year-old inmate, Dwayne Stafford, who is black, to punch Roof a number of times in the face and back. Officials were unaware of any verbal exchanges between the two inmates in the past, he said.
Prosecutors have said Roof's attack on the historic Emanuel African Methodist Episcopal Church in June 2015 was racially motivated.
On Thursday, two detention officers quickly responded and separated the inmates, Cannon said.
The church shooting suspect is being held in a protective custody unit at the Sheriff Al Cannon Detention Center and is not supposed to have any interaction with other inmates.
Cannon said his office will investigate how Stafford was able to exit his cell, which should have been locked while Roof was showering.
The sheriff said he also wanted to know why one detention officer was taking a break while another responded to an inmate who needed toilet paper, leaving Roof unguarded.
"At the very least, we have complacency, which for police officers and detention officers is one of the biggest challenges we face," the sheriff said. "This certainly serves as a wakeup call."
The sheriff's office issued an incident report that said an arrest warrant would be sought to charge Stafford with assault and battery, but Cannon later said Roof and his lawyer had indicated they would not pursue charges.
Prosecutors have charged Roof with dozens of offences in both state and federal courts, including murder and hate crimes, and could face the death penalty. The federal trial is set to begin November 7.
Court records filed by prosecutors in federal court in July said Roof used eight Glock handgun magazines and fired about 80 rounds during the massacre which he had plotted for months.
Roof's attorneys argued in U.S. court papers filed this week that he should not face the federal death penalty, which they deemed "cruel and unusual punishment" and, as a result, unconstitutional.
© Thomson Reuters 2016