A 27-year-old District man pleaded guilty yesterday to second-degree murder, admitting in D.C. Superior Court that he was involved in the robbery that led to last January's slaying of a D.C. police sergeant but said that he did not fire the gun that killed Joseph M. Cournoyer.

Charles A. Blackwell, charged with first-degree murder, pleaded guilty to lesser charges of second-degree murder while armed during proceedings that were intently watched by Cournoyer's widow and later criticized by his colleagues as too lenient.

Blackwell, a man described in court papers as "borderline retarded" and by his attorney as a "pawn in someone else's game," quietly answered "yes" when Chief Judge H. Carl Moultrie I, in a series of questions, asked if he agreed with the prosecution's account of how the 30-year-old officer was fatally shot at the Minnesota Avenue Metrorail stop shortly after Blackwell and two other men robbed a nearby meat store.

Police said Cournoyer was shot by one of the suspects he was trying to take into custody. The man charged with shooting Cournoyer is awaiting trial.

Blackwell made no statement. He often appeared confused when Moultrie asked if he understood the proceedings. Later, his attorney Roger Durban said Blackwell, too, "was a victim in his own right."

"The bottom line," said Durban, "is that Blackwell came here and with a very limited intellectual capacity and with the environment over there in the projects got . . . swept away by an attitude a lot of young guys over there have . . . . The only person in the District who grieves more deeply today about what happened than Charles Blackwell is Cournoyer's widow."

Darlene Cournoyer sat in the courtroom's second row and stared directly at Blackwell. "I can't say if I'm satisfied until I hear the sentencing," said Cournoyer.

In addition to the murder charge, Blackwell pleaded guilty to armed robbery. Each charge carries a maximum life sentence.

In exchange for the plea, the government agreed to dismiss a first-degree murder while armed charge as well as other charges related to the Jan. 29 robbery of Murry's Steaks at 4061 Minnesota Ave. NE, and Cournoyer's subsequent slaying. Had Blackwell gone to trial, he would have faced a mandatory sentence of 20 years before he would become eligible for parole. Under his plea agreement, Blackwell, of 3963 J St. NE, must be sentenced to at least one mandatory five-year jail term and could be sentenced to two mandatory five-year terms because he was armed.

A second man accused in the slaying and identified in court papers as the lookout man, David A. Corbin, earlier pleaded guilty to second-degree murder and armed robbery. A third man, Sylvester R. King Jr., is being held without bond on a felony murder charge. Police allege that King shot Cournoyer.

Prosecutor Alan Strasser told Moultrie yesterday that the government was prepared to prove that Blackwell and King boarded a bus at the Minnesota Avenue Metrorail station after the hold-up. Cournoyer boarded the back of the bus and attempted to escort King off the bus, Strasser said. A struggle ensued and Cournoyer was shot just below the heart.

Yesterday, Cournoyer's colleagues said they were disturbed by the plea agreement.

"It's not good enough," said officer Jim Murphy.

"I understand what the prosecutor is trying to do . . . but his death was rotten and the way the judicial system works is rotten," said Sgt. Michael McNeely, Cournoyer's partner. McNeely compared Blackwell's plea to this week's guilty plea by a man in the April slaying of a Virginia state trooper. The man pleaded guilty to capital murder and the judge must sentence him to death or life imprisonment without possibility of parole.

U.S. Attorney Joseph diGenova predicted it would be a "very, very long time" before Blackwell would be released from jail. Blackwell's sentencing is set for Sept. 6.