Dylann Roof hesitated for about 20 seconds when an FBI agent asked him what he was doing on the night nine black church members were killed during Bible study, and then admitted: "I did it."
In a video recording of his interview, played in public for the first time today at his death-penalty trial, he was then pressed by the agents and asked exactly what he did.
He paused for another 30 seconds or so, and then said: "I killed them."
As he talked more, he chuckled and said: "Well, I killed them, I guess."
The video was shown on the third day of evidence at his trial, where Roof is accused of opening fire inside a room at the Emanuel African Methodist Episcopal Church in Charleston, South Carolina, on June 17, 2015, just as members of the Bible study class closed their eyes for a final prayer.
Roof, 22, told the agents he did not start firing when he first entered the room.
"I was sitting there thinking about whether I should do it or not. That's why I sat there for 15 minutes. I could have walked out," Roof said.
Church surveillance video indicate Roof was inside for 45 minutes. A survivor has told the court he was given a Bible and a study guide to follow along with the prayer group.
Roof, as he has for much of the trial, hardly looked up as the confession played, mostly just shuffling papers in front of him at the defence table.
FBI agent Michael Stansbury said he pushed Roof to confess so quickly because he sensed he wanted to talk.
"He was calm, he wasn't upset," Mr Stansbury said.
Roof is charged with 33 federal counts, including hate crimes.
His defence has largely conceded that he committed the killings and has instead focused on trying to spare him the death penalty.
If jurors at the Charleston court find Roof guilty, they will decide whether he should be put to death or spend the rest of his life in prison.
Roof has said he wants to represent himself during that penalty phase of the trial.
In the confession, Roof said he left bullets in a magazine so that he could kill himself after the shootings but changed his mind.
At one point, an agent asked if Roof thought about killing more blacks.
"Oh no. I was worn out," Roof said.
He said he chose Emanuel in part because it is the oldest black church in the South and that the killing of Trayvon Martin was a turning point in his life.
Mr Martin, a young unarmed black man, was killed by neighbourhood watch volunteer George Zimmerman in 2012. Zimmerman was acquitted in the shooting.
Roof can be seen laughing occasionally throughout the questioning. About 45 minutes into it, agents finally tell Roof that he killed nine people.
"There wasn't even that many people in there," Roof said incredulously. "Are you lying to me?"
The only other glimpse into Roof's motivation is a 2,000-word statement he posted online on the afternoon of the shooting and 60 photos he carefully picked from more than 1,000 he had taken. Some of the photos included Roof posing with the Confederate flag.
In Roof's essay, he said he thought blacks were stupid, inferior to whites and violent. Among other things, he wrote: "We have no skinheads, no real KKK, no-one doing anything but talking on the internet.
"Well someone has to have the bravery to take it to the real world, and I guess that has to be me."