A 15-year-old District youth turned himself in yesterday and pleaded guilty to felony murder in the slaying of a popular waiter in the Dupont Circle area.

The youth, whose name was withheld because he was charged as a juvenile, is the second person to be arrested in the killing of the waiter, Adrien D. Alstad, as he walked home from his restaurant Aug. 23. Police said Alstad was shot during an attempted robbery.

The other person charged, Kurt D. Williams, 19, has been in custody since his arrest two days after the killing. He is awaiting trial on a first-degree murder charge.

Soon after turning himself in to authorities, the juvenile appeared in D.C. Superior Court, where he accepted responsibility for Alstad's death. The hearing was closed to reporters. Under a plea agreement, the youth admitted to being the gunman, authorities said.

Because he was charged as a juvenile, the longest that the youth can be confined is until age 21. Had he been charged as an adult, he could have faced a life prison term. The earliest age at which juveniles can be tried as adults in the District is 15. But before a 15-year-old can be moved into the adult system, D.C. law requires prosecutors to prove that the youth is a threat to public security and has no reasonable prospects for rehabilitation in the juvenile justice system.

The plea agreement was disclosed by D.C. Deputy Attorney General David M. Rubenstein after the hearing. He said the youth agreed to plead guilty to murder and other charges after authorities "learned he was the shooter." As a result of the agreement, the teenager was not charged as an adult, Rubenstein said.

Alstad, 55, was a popular waiter at Annie's Paramount Steak House in the 1600 block of 17th Street NW. He was walking home about 2:20 a.m. Aug. 23 in the 1800 block of R Street NW when two assailants approached and one shot him, police said.

The break in the case came two days later, when police arrested Williams and two juveniles just minutes after a robbery that occurred about 12 blocks from the homicide scene, police said. One of the juveniles was the 15-year-old, who identified Williams as the gunman in Alstad's slaying, police said.

Detectives used that statement to file the first-degree murder charge against Williams, police said.

Williams also implicated himself in Alstad's attempted robbery and killing, telling detectives that Alstad was shot accidentally while he and the waiter fought over the gun, police said.

Detectives initially suspected that the 15-year-old might have been involved in Alstad's death, but they did not believe at the time that they had enough evidence to charge him with murder, police said. But later, the youth failed a polygraph test and admitted to being the gunman, police said.

Leigh Hendricks, manager at Annie's Paramount Steak House, where the personable Alstad entertained customers by occasionally breaking into song, said she was upset that prosecutors did not charge the youth as an adult.

"I thought they had found the person that shot and killed Adrien," Hendricks said, referring to Williams. "And now we're finding out that it was someone with him, so he'll serve, what, maybe six years. . . . I'm not happy about it. No one is."

Staff writer Henri E. Cauvin contributed to this report.