The agents arrested Joseph "Joey" Meek Jr. while he was at work, his girlfriend, Lindsey Fry, told The State newspaper.
A anonymous federal law enforcement official told The Associated Press on Wednesday that Meek had been informed last month in a letter that he was under investigation for lying to law enforcement and misprision of a felony. That suggests authorities believe Meek allegedly knew about a crime before or after it was committed, but didn’t report it.
Meek did speak to investigators in the wake of the shooting. In a previous interview with the AP, he said he called the FBI as soon as he recognized Roof in surveillance video from the shooting.
He also told the AP that he and Roof, who attended high school together, had reconnected in the weeks before the shooting. Meek said that Roof complained to him that "blacks were taking over the world" and spoke of a "plan" after buying a .45-caliber Glock pistol with his birthday money.
This post has been updated.