An 18-year-old Northwest Washington man who was shot by a D.C. police sergeant Tuesday near one of the District's most notorious illegal drug markets remained in critical condition yesterday at the Washington Hospital Center's MedStar Unit, a spokeswoman there said.

Police said Joseph Porter of 30 T St. NW was shot shortly after fighting with Sgt. Manuel G. Rivera in the back yard of a building in the 1300 block of North Capitol Street, near Hanover Place NW.

Sources familiar with the police investigation of the incident said Porter was attempting to flee at the time Rivera shot him, and that just before the shooting Porter had appeared to reach for his waistband as if attempting to pull out a gun. No gun has been found, police said.

The bullet that hit Porter in the leg severed a major artery, hospital officials said.

Sources familiar with the investigation said Rivera has told D.C. homicide investigators that he was on routine patrol searching for illegal drug transactions about 5 p.m. when he saw two persons in an alley that runs between O Street and Hanover Place, known as a drug market. They fled when they saw the officer, the sources said.

Rivera has told investigators that he drove into the alley and, as he got out of his car, noticed that one of the persons had what appeared to be a handgun in his waistband, the sources said. When he attempted to arrest the man a fight ensued, during which Rivera was knocked to the ground, the sources said.

Rivera has told investigators that the man then started running away, and while fleeing, appeared to reach into his waistband as if readying himself to turn and fire, the sources said. They said it was then, while lying on the ground, that Rivera drew his service revolver and fired.

Persons who claim to have been at, or near, the shooting scene have told the investigators that Porter was not armed, and police said yesterday that no weapon has been recovered.

A mud-caked toy gun was found in the alley, the sources said, and police are investigating whether it was there earlier, or dropped after the shooting.

Porter, who according to court documents was charged with two counts of distributing cocaine and possession with intent to distribute cocaine in an unrelated incident two weeks ago, was charged with assault on a police officer with a dangerous weapon in connection with the incident Tuesday.

Police said that no drugs were confiscated after the shooting.

Rivera has been involved in two previous shootings, police sources said, including the fatal shooting of 36-year-old Jeanette G. Boykin in Northwest Washington two years ago. The sources said that no wrongdoing was found in either shooting, and that no disciplinary action was taken against the officer.

He will continue with his duties as a patrol sergeant while the investigation of Porter's shooting continues, a police spokesman said.