At first, Cornelia Robertson didn't believe the caller who told her that her teenage son was dead in a Southeast Washington alley.

Then, as the person, a friend of her son's, grew more specific and refused to back down, Robertson began to panic.

She called police. Within minutes, her worst fears were confirmed: Police went to the scene and found her 15-year-old son, Shawn J. Riley, slumped in the back seat of a stolen car with gunshot wounds. He died shortly afterward at Howard University Hospital.

"I was devastated," Robertson said yesterday. "I was in shock. I didn't want to believe that it was really happening."

Her son's death late Monday pushed the number of juveniles slain in the District this year to 22. Last year, a total of 12 people younger than 18 were killed in the city.

Police said yesterday that they had no motive or suspect in the case and that they were led to the homicide scene only by Robertson's phone call.

When police discovered the stolen car about 11 p.m. Monday in the rear of the 700 block of Brandywine Street SE, the engine was running and one of the back windows had been blasted out by gunshots, authorities said. The car was parked next to a dumpster in a dark alley behind a row of apartment buildings.

Detectives are not sure whether the killing occurred in the alley or elsewhere, police said.

Riley's mother and police said the person who called her did not witness the shooting. Detectives were seeking him for questioning, police said.

Investigators said they believe that others were in the car with Riley when the shooting occurred. The car, a 1994 Oldsmobile, was reported stolen from the 700 block of Mississippi Avenue SE about an hour before it was discovered, police said.

Robertson said Riley's life had been troubled in recent years.

His older brother, 19-year-old Milan H. Lucas Jr., was killed in July 2000 in a double shooting in Southeast Washington; a few weeks later, his best friend died in a traffic accident, Robertson said. Soon Riley was suffering terrible headaches, struggling in school and running afoul of police, she said.

Doctors told Robertson that her son was suffering from stress related to the death of his brother and friends. The boy told doctors that 13 buddies had been killed, Robertson said.

"It was just too much for him," Robertson said.

Riley, who had been released this year from probation in a drug case, loved cars and staying out late, often coming home after 2 a.m., his mother said.

Police cited him several times for driving without a license and picked him up for violating the city curfew on several occasions, she said. Sometimes he would vanish for days at a time, his mother said. He had chosen the wrong crowd of friends, Robertson said, adding that she warned him repeatedly to shape up.

"I told him before that he didn't have any friends out here," said Robertson, 53. "I told him: 'Who comes to court for you? It's me. When you are picked up by police, who comes down to see you? It's me. Your friends didn't come down to visit you. It was me who came down to get you out.' "

Riley liked football and video games and other typical teenage pursuits, she said. The family recently returned from a trip to Disney World. He enjoyed some classes at school, particularly reading and math, his mother said. But he struggled in others, leading him to skip classes from time to time at Eastern Senior High School, where he was a ninth-grader, his mother said.

School officials did not respond to requests for interviews yesterday.

Students at the high school, on East Capitol Street, said they learned about the killing at the end of the school day when an announcement was made over the public address system.

Riley was the sixth juvenile to be killed in a car in the District this year and the 16th to die by gunfire. The spate of slayings has confounded police because it is taking place despite an overall decline in killings. The District is on pace to record about 200 killings this year, which would be the lowest number in nearly two decades.

Nationally, some large cities have reported similar surges in juvenile deaths this year.

Shawn J. Riley is shown with his mother, Cornelia Robertson, and sister Crystal Robertson. He was found fatally shot in a stolen car Monday night, police say.