A man who pleaded guilty to murder after firing an assault rifle into a crowd of mourners two years ago spoke at length publicly Wednesday for the first time since his arrest, expressing remorse for his role in a string of shootings that left five dead in the District.

“I played a major part in what was done,” Nathaniel Simms said during testimony in D.C. Superior Court. “I’m looking to spend the rest, if not all, of my life in jail. To expect any less would be dumb and naive.”

Simms, 28, was arrested in the aftermath of a drive-by shooting on March 30, 2010, that capped eight days of violence in the city. Originally charged with first-degree murder, he pleaded guilty a month later to five counts of second-degree murder and two counts of conspiracy to commit murder, and agreed to testify against five other men.

The trial of those men — Sanquan Carter, 21; Orlando Carter, 22; Jeffrey Best, 23; Robert Bost, 23; and Lamar Williams, 23 — on first-degree murder and other charges is now in its third week.

Simms faces at least 25 years in prison and could spend the rest of his life behind bars. On Wednesday, he told Assistant U.S. Attorney Michael Brittin he agreed to testify to take responsibility for his actions, not to curry favor with prosecutors or the judge who will sentence him.

“I knew what I did was wrong,” Simms said on the stand, his voice cracking. “Every night, it’s all I really think about.”

Speaking deliberately, Simms displayed little emotion as he recounted the early hours of March 22, 2010, when, he and prosecutors say, a bracelet belonging to Sanquan Carter disappeared at a party in the 1300 block of Alabama Avenue SE.

The shooting that resulted left Jordan Howe, 20, dead and two others injured, authorities say. Simms and four of the other defendants were allegedly involved in two other shootings eight days later: One left 17-year-old Tavon Nelson dead and the second, minutes later, killed three more.

In court Wednesday, Brittin asked Simms if he knew the names of the three people who died in the final shooting along South Capitol Street in Southeast. Simms admitted to opening fire on a crowd — many had attended Howe’s funeral earlier in the day — with an AK-47-style assault rifle from a moving minivan.

Simms recited them: Brishell Jones, 16, DeVaughn Boyd, 18, and William Jones III, 19. Bri­shell’s mother rushed from the courtroom as he did.

Hours before the March 22 shooting, Simms testified, he was gambling with Best and Orlando Carter. As he drove them home, Carter’s cellphone rang: It was his brother Sanquan, who said that his gold-colored bracelet had been stolen while he was at the Alabama Avenue party.

Simms said Orlando Carter took the wheel and drove to the home of a woman said to be Carter’s “godmother.” He went to the door, Simms said, and then returned with the assault rifle inside his coat.

Brittin held up the rifle, a brown leather shoulder strap dangling from it, in the courtroom. Some of the victims’ family members ran weeping from the room when they saw it.

Orlando Carter then drove to Williams’s home, Simms said, where Williams produced a pump-action shotgun and a .380 semiautomatic handgun.

“They robbed my little brother. They gonna see,” Simms said Orlando Carter told Williams before they left him. Best jumped back and forth in the back seat, yelling “I love this [expletive]. I love this [expletive],” Simms said.

When they arrived at the Alabama Avenue apartment building, Simms said, Orlando Carter ordered him into the driver’s seat. “Orlando was heated and mad,” Simms said.

Best’s mood had evidently changed: “Jeff was calm and chill,” Simms said.

Simms said he knew what would follow. “I knew if we ran into who we were looking for, someone would get shot or possibly killed,” he said.

From inside the car, Simms said, he saw Sanquan Carter pat down several people standing outside the apartment building. Then, Simms said, Sanquan Carter grabbed the .380 handgun, Orlando Carter wielded the assault rifle and Best took the shotgun, and they began shooting.

Sanquan Carter was the first to fire, Simms said. “I heard a lot of shots and saw smoke coming off the building,” he said.

Simms said that he had had long friendships with the five defendants, who he said were neighborhood friends. He had known some since 2003, and Brittin showed photographs of Simms and several of the men drinking and flashing neighborhood signs at nightclubs. In one photograph, some stood alongside Simms in front of a poster for the 1983 Al Pacino gangster movie “Scarface.”

As Simms talked, the five men watched emotionless from seats behind their attorneys — and flanked by federal marshals — from the other side of the courtroom.

Simms testified for about two hours Wednesday. He is expected to continue Thursday.