LORTON, Va. (AP) _ One inmate was killed and another injured as fires set by prisoners under the cover of a power failure destroyed an administration building at a troubled District of Columbia prison complex, authorities said.

Firefighters from Fairfax County, Va., let the flames spread unhindered for at least an hour late Friday night because inmates were roaming free within the Lorton Reformatory's grounds, fire officials said.

Washington, D.C., Mayor Marion S. Barry Jr., who came to the city-run prison complex in southern Fairfax County early Saturday, downplayed the disturbance.

''I think this was a minor situation. I think the department did an excellent job, but you have people in jail who do not want to be there,'' Barry said.

At least four fires were set by inmates, according to Lt. Mike Reilly, spokesman for the Fairfax County fire department. One blaze destroyed the main administration building for the medium-security central facility, which houses about 1,280 male inmates in dormitories and has no individual cells.

The prison - one of 10 correctional institutions in the Lorton complex - is about 20 miles south of Washington and held 6,165 prisoners last month.

Police from Fairfax County and the district and Virginia State Police ringed the outer fences to guard against escape attempts, county officials said. All inmates were accounted for and the facility declared secure about four hours after the fires started.

Inmates started the fires almost immediately after a power failure shut down all lights at the complex about 9:15 p.m., corrections department spokeswoman Pat Wheeler said.

Members of the D.C. police department's special operations division and civil disturbance unit - some on overtime duty because of the events surrounding President Bush's inauguration - rushed to the scene, as did about a half dozen D.C. fire department units, police said.

It was 3:30 a.m. before the last of the fires was declared under control, county fire officials said.

Ms. Wheeler said only one of the 25 dormitories was damaged. She said 60 inmates were transferred to the district's jail fire damaged the dormitory's heating system.

Officials did not release details about the inmate's death or about injuries to the other prisoner. The injured inmate was taken to D.C. General Hospital but no information about his condition was released.

The disturbance was the latest in a series of problems plaguing the city prison system, including escapes, accusations of drug dealing inside prison walls, disorders and years of litigation to reduce overcrowding. Several judges have placed limits on city prison facilities and a federal judge in 1987 found the city in contempt for violating a cap at one Lorton site.

But corrections director Hallem Williams said early today the disturbance was not caused by overcrowding. ''We are in compliance with the cap and overcrowding was not the issue,'' he said.

The city closed its doors to new inmates for six weeks last fall, opening them again only under a judge's order. The federal government was forced to take charge of 300 city inmates during that time.

The district has also sent inmates to the states of Washington and Tennessee to help relieve overcrowding.

Last year, 18 inmates fled from the Lorton complex or from work-release facilities connected with it. Most have been recaptured, but the escapes exacerbated Fairfax County officials' opposition to the facility.

In July 1986, fires set by inmates destroyed four dormitories at the Occoquan area of the correctional complex. That blaze was blamed for severely intensifying the crowding that has plagued the prison system.

In 1985, nine inmates at Lorton were wounded when guards fired shotgun blasts into a crowd of rioting prisoners who were armed with bricks, bottles and homemade knives. Those inmates were also housed at the central facility.

Rep. Stan Parris, R-Va., a vocal critic of the district's management of Lorton, introduced legislation in Congress last fall to give Virginia some control over the prison complex. He said then, ''The situation at Lorton reached crisis proportions a decade ago, and it has continued to deteriorate.''