Two police officers who shot and killed a 21-year-old Mount Vernon man by mistake last month have been cleared of violating police procedures in the incident, although an investigation by the Fairfax County prosecutor is continuing.

Fairfax County police Sgt. Walter Blankenship, 38, and Officer Kenneth Madden, 33, of the Arlington police force were cleared by an internal review by the Fairfax police department, Fairfax Chief Richard A. King said yesterday.

Fairfax Commonwealth's Attorney Robert F. Horan Jr. is still investigating to determine if there were criminal violations in the death of Stanley Hughes, who was shot to death Feb. 12 as he ran unarmed from his apartment where a murder suspect was staying. Horan refused comment yesterday on the status of his investigation.

Blankenship, who had been suspended from the force, returned to his job Monday at the Groveton police substation. Arlington police said Madden, assigned desk duty pending the Fairfax police department review, would be back on street patrol by today.

The officers who shot Hughes had been staking out Hughes' apartment at 3332 Lockheed Blvd. in the Mount Vernon area in hope of capturing Kenneth Eugene King 34, who was sought in connection with the death of his father-in-law and the wounding of his mother.

Hughes was shot as he ran from his apartment after gunifire was heard from outside. Afterward, police said, they found King inside the apartment dead of a self-inflicted gunshot wound.

At first, the police effort to investigate the matter was frustrated, first by the officers' refusal, on advice of their attorneys, to discuss the matter, and then by Hughes' mother, Naomi Hughes, the only other eyewitness to the incident, who announced she would have "nothing to say to police."

Blankenship and Madden decided about two weeks ago to talk with police investigators about the incident.