A 19-year-old prisoner at the District's Youth Study Center in Lorton was stabbed to death yesterday morning by an inmate armed with a homemade knife, authorities said.

It was not clear what prompted the stabbing, which occurred at 8:35 a.m., but Fairfax County police investigating the killing said they have a suspect and charges are pending.

The victim, Gordon Stewart, was stabbed in the chest while in the shower area at Youth Center I, one of two facilities for young offenders from the District at the Lorton Correctional Complex. He was taken to the center's clinic, where he was pronounced dead at 8:45 a.m.

Unlike the rest of the buildings in the complex, which are on federal land, the two youth center buildings are in Fairfax County and fall under the jurisdiction of the county police.

Stewart was serving a three-year sentence for armed robbery and one year for a probation violation, a spokeswoman for the D.C. Department of Corrections said. He had been at the center since June 1988.

No information on the suspect was released yesterday. Inmates at the center are sentenced under the Youth Rehabilitation Act, and they all are 18 to 21 years old, the spokeswoman said.

The last homicide at the center occurred in February 1988, when an inmate, Antonio Bowers, was stabbed to death.