Fairfax County and Arlington police mistakenly shot and killed a 21-year-old Mount Vernon man early yesterday as he ran unarmed from his apartment where a murder suspect was staying.

Police said they opened fire killing Stanley Hughes yesterday because he resembled the suspect. Hughes' death climaxed a series of shootings that started Saturday in which three persons died and one was wounded.

The police officers who shot Hughes were staking out the apartment at at 3332 Lockhead Blvd. in hope of capturing Kenneth Eugene King, 34, who was sought in connection with the shooting death of King's father-in-law Sunday and the wounding of his mother-in-law Saturday.

After Hughes was mistakenly shot, police said they entered his apartment and found King dead of a self-inflicted gunshot wound in the chest.

Fairfax police said an internal investigation is being conducted into Hughes' death. The Fairfax police officer involved, Sgt. Walter Blanken-ship, was placed on administrative leave pending the outcome.

The status of Arlington police officer Kenneth Madden, who also fired at Hughes, had not been decided last night, an Arlington police spokesman said.

The shooting of Hughes comes at a time of growing concern over the way in which Fairfax County handles its law enforcement. A community group in the same general area of the county where yesterday's shooting occurred conducted hearings recently into allegations of police brutality.

The County Board of Supervisors has also expressed concern over the shooting death of a private school custodian who was mistakenly killed during a police stake-out Dec. 29 and the death of a Gum Springs man after a four-day stay in the county jail.

The supervisors yesterday requested a "full report" from its staff on the death of the custodian, John Jackson. Officer David Lubas, the policeman who shot Jackson three times, has been cleared of any wrongdoing by an internal police investigation.

The events that led to the fatal shooting of Hughes early yesterday began Saturday when King's mother-in-law, Mabel Wright, was shot and wounded in the neck at her home at 1429 S. Rolfe St., Alexandria.

King was suspected of that shooting and of the fatal shooting the following day of his father-in-law, Johnny Lee Wright, 57, whose body was found on the street in the 500 block of N. Imboden St., Alexandria.

Police said they went to the white brick garden apartment building on Lockheed Boulevard early yesterday because they had learned that King was in Hughes' second floor apartment.

After police telephoned the apartment attempting unsuccessfully to persuade King to surrender, shots were heard from inside the apartment, Arlington police said.

At that point a woman -- later identified as Hughes' mother, with whom he lives -- ran screaming from the apartment, followed, police said, by a man fitting the suspect's descripton. The two officers stationed immediately outside the apartment door opened fire, according to police.

Police said King, described as 5 feet 10 and weighing 180 pounds, was believed related to Hughes, described by a neighbor, Glenn A. Russell, as about 5 feet 8 and "slim."

Hughes' mother was reported under sedation last night.

Police said that, pending an autopsy, it was unclear how many times Hughes had been shot. It was also unclear whether he was struck by bullets fired by both officers or only one. Ballistic tests are to be made on any bullets recovered in an effort to match them with the gun or guns that fired them.

Fairfax County Commonwealth's Attorney Rabert F. Horan Jr. said he will review the results of the internal police investigation of Hughes' death and investigate further if neecessary.

But the prosecutor added, "Statistically, it's not fair to say that this department shoots first and asks questions later... There's less use of weapons by this department than by any other department I'm aware of."

Fairfax County Deputy Police Chief Kenneth R. Wilson said last night of Hughes' death, "We are just very displeased that this has occurred, [but] I don't want to be critical of anybody until this investigation" is complete.