A man who prosecutors said acted as the lookout in a Northeast Washington robbery that culminated with the slaying of a D.C. police sergeant in January pleaded guilty yesterday to second degree murder and armed robbery charges.

He also agreed to testify against two codefendants in D.C. Superior Court.

David A. Corbin, 20, of 3519 Jay St. NE pleaded guilty before Chief Judge Carl H. Moultrie I in the slaying Jan. 29 of Sgt. Joseph M. Cournoyer.

Cournoyer was shot as he tried to detain a man at a Northeast bus stop for questioning in connection with the robbery of a nearby store.

Assistant U.S. Attorney Alan Strasser told Judge Moultrie that if the case were tried, prosecutors were prepared to show that Corbin acted as a lookout outside the store, took a bag of money from one of the robbers and fled with them to the bus stop where Cournoyer was fatally shot moments after the robbery.

U.S. Attorney Joseph E. diGenova said Corbin could face a maximum penalty of 30 years to life on the charges.

"This sends a message to would-be lookouts and getaway drivers, that they are subject to the same penalties as individuals who pull the trigger," diGenova said after the court hearing.

Corbin, who prosecutors said will testify at the trials of two codefendants, was placed in protective custody after the court hearing.