A 20-year-old inmate at Lorton Reformatory's Youth Center No. 1 was fatally stabbed Wednesday night during an incident behind the facility's admissions building. Fairfax County police said yesterday they have questioned a suspect in the case and expect to charge him today with the slaying.

Thomas Watts of 1811 Ninth St. NW, who had been at the District-operated facility since June 1983, was stabbed 11 times about 8:30 p.m. Wednesday, according to LeRoy Anderson, D.C. Department of Corrections spokesman.

Watts was taken to DeWitt Army Hospital, where he died about 90 minutes later, Anderson said. Watts was serving a six-year sentence for his part in a 1983 armed robbery in Northwest Washington.

Anderson said that five inmates were locked in segregation after the incident and questioned by Fairfax County police, who have jurisdiction over the youth facility, which is part of a large prison complex in southern Fairfax County.

County police spokesman Warren R. Carmichael said that police expect to charge a 20-year-old inmate from Maryland in connection with the stabbing. He said charges also may be brought against other inmates.

Anderson said that Watts was found after a corrections officer in a tower reported to the facility's command center that he saw a "scuffle" involving about six inmates who had gathered behind the admissions building. Five officers dispatched to the area found Watts lying on the ground and attempted to revive him, according to Anderson.

Carmichael said the county was "notified promptly" of the incident and that officers had found a pair of scissors at the scene.

Anderson said that corrections officers yesterday found a knife with what appeared to be blood on it in a ditch near the facility's school, about 200 yards from where the stabbing occurred. It was unclear if the knife had been used in Watts' stabbing, he said.

The last fatal stabbing of a Lorton inmate occurred on Dec. 28, 1984, when Thermond Creasey was killed in the institution's Central facility. No one has been charged in that incident, according to Anderson.