Two weeks after Stanley Hughes was accidentally killed by Northern Virginia police, the officers trying to discover why he was shot say they know little more about the incident than they did on the day of the shooting.

The reason: the two police officers who fired shots at Hughes are refusing to talk to investigators -- as is the only other witness to the shooting.

What is thwarting the Fairfax County investigation is part of what police officials say is a national trend. Increasing numbers of officers who become the subjects of internal police probes are invoking their constitutional right to remain silent, the right that the officers routinely offer every criminal suspect.

In Fairfax County, top low enforcement officials are troubled by the officers' silence in the Hughes shooting, but they admit that, for the moment at least, they are powerless to act.

"I flat do not understand it," says Fairfax County prosecutor Robert F. Horan Jr. "We have had many internal police investigations and I can never recall a shooting case where the officers involved did not cooperate."

Hughes 21, was killed in the predawn of Feb. 12 as he dashed unarmed in night clothes from a Hybla Valley apartment he shared with his mother, Naomi Hughes. Moments earlier a 34-year-old murder suspect whom Fairfax and Arlington police were seeking has shot himself in the Hughes' apartment.

Neither Arlington police officer Kenneth Madden nor Fairfax Sgt. Walter Blankenship, identified by police as the two officers who fired four shots at Hughes, is talking.

That's forcing a delay in the Hughes investigation that one top Fairfax officer calls "embarrassing." When the officers' lawyers got "into the act" the investigation slowed "considerably," said Deputy Fairfax Police Chief Kenneth R. Wilson. "We are displeased." wilson is not alone is his frustration, according to Glenn Murphy, general counsel for the International Association of Chiefs of Police. "A police officer charged witha potential criminal incident has the same rights as any other citizen. But when he invokes the public thinks that is terrible and generally so does management," Murphy said.

Last year in Prince George's County, investigations into two separate police shootings dragged on for several months after the officers involved hired their own lawyers, a county police spokesman says.

Montgomery County lawyer Allan Katz, who is assisting an officer under investigation there, says "there is very little advantage to being overly talkative. You might say something innocently and it might be misinterpreted."

What troubles many police administrators, however, is a fear that the silence of their officers may undermine public confidence in the police.Last week, sa if to assure the public that te force was still pressing the Hughes investigation, the Fairfax police issued a statement, saying that they had nothing new on the case, largely because of the officers' silence.

Hughes was preceded by his mother when they ran out of the apartment. Naomi Hughes, whom olice say is the only other witness to her son's death, isn't talking to police either.

She has hired a lawyer, James Raby, who says talkin to police could hurt her chances of any future legal acton over the killing.

Fairfax investigators say they are hopeful that the two officers will reconsider their lawyer's advice.

Faced with the two officers' refusal to aid police, Fairfax Police Chief Richard A. King said yesterday he will turn the matter over -- probably this week -- to Horan. If the officers are not faced with criminal charges, they would then be required to cooperate in an administrative police inquiry or face dismissal, King said.

Blankenship has been placed on administrative leave and Madden has been assignd a desk job in Arlington while the investigation continues.