Dylann Roof drove to another African-American church after killing nine black parishioners in Charleston, South Carolina.
In newly unsealed court documents, U.S. prosecutors said the move was a sign he intended to carry out more racially-motivated attacks.
Roof was sentenced to death in federal court in January after being found guilty of charges connected to the June 17, 2015, massacre at a Bible study meeting at Emanuel African Methodist Episcopal Church.
In a September court filing released on Tuesday, federal prosecutors said that after the mass shooting, Roof fled the church with his gun and drove about 20 miles to Branch AME Church in Summerville.
That church also has a predominantly African-American congregation and held a Bible study meeting on the night Roof drove to it. GPS evidence from Roof's car showed he slowed and then stopped at the church for two to three minutes, according to prosecutors.
The U.S. government said the similarities between the two churches suggested Roof "intended to continue his racially-motivated violence at Branch AME Church that night and, more specifically, that his intended targets were African-American congregants at a church."
Roof told Federal Bureau of Investigation officials after his arrest that he was “worn out” by the Emanuel shooting and did not plan to commit more killings after he fled Charleston.
The court filing said GPS evidence showed Roof had visited the Branch AME Church before, scouting it out in February 2015 as he prepared for an attack.
Prosecutors said Roof admitted he researched various targets, including other African-American churches and a “black festival” before choosing Emanuel AME Church for the shooting.During Roof's trial, prosecutors showed jurors a list of black churches in South Carolina that investigators had found in a backpack in his car after his arrest.
That list included Emanuel AME Church but not the Branch congregation, according to the newly unsealed court documents.
Roof, who earlier this month made a motion for a new federal trial, also faces a second death sentence if he is convicted of murder charges in state court. No trial date has been set.
(Reporting by Harriet McLeod; Editing by Colleen Jenkins and Tom Brown)