Roof's competency was the subject of a 16-hour hearing held behind closed doors on Monday and Tuesday. Earlier this month, defense attorneys had raised concerns for the first time about whether Roof was able to understand the nature of the proceedings against him and to assist in his defense.
U.S. District Judge Richard Gergel said in an order on Friday that he had carefully considered the testimony of a renowned psychiatrist who evaluated Roof, 22, as well as arguments from lawyers.
The judge kept under seal his detailed findings about Roof's competency, saying that making them public would jeopardize the defendant's right to a fair trial.
Jury selection now will resume on Monday.
Roof faces 33 counts of hate crimes, obstruction of religion and firearms charges in the massacre at Emanuel African Methodist Episcopal Church in Charleston, South Carolina in June 2015.