About 400 inmates who were evacuated from a Lorton Reformatory facility last month after methane gas explosions fatally injured one inmate and critically burned another may be returned to the facility by the end of this month, officials said yesterday.

D.C. and Fairfax County spokesmen, as well as specialists investigating the source of the gas, confirmed yesterday that the methane gas came from a nearby landfill. They said methane specialists are designing systems to stop the gas from seeping into the facility.

"There is more than reasonable certainty that the methane is coming from the landfill," D.C. City Administrator Thomas Downs said yesterday. "The uncertainty is how it's traveling through the ground."

Inmates at Youth Center No. 1 were transferred to other institutions at the District's sprawling prison complex in southern Fairfax County when officials concluded that explosions on Dec. 3 and Dec. 6 were fueled by gas seeping into the facility. The transfers caused severe overcrowding and heightened tension at the other facilities where sporadic outbreaks of violence were reported.

For more than three weeks, Fairfax County public works officials, prison authorities and methane gas specialists have been drilling wells and monitoring gas levels in the ground at and near the facility to determine the source of the leak.

Youth Center No. 1 is about 200 feet from the 300-acre I-95 Landfill, which handles more than 4,000 tons of refuse daily from the District, Fairfax County, Arlington and Alexandria.

According to a 1982 study by methane specialists at SCS Engineers in Reston, the landfill produces about 500,000 cubic feet of gas every day.

Experts investigating the leak said that they have ruled out every other source but the landfill, and SCS Engineers is currently designing a "double protection system" to stop the gas from seeping into the facility, sources said.

Officials familar with the plans said that the system will include a gas extraction system at the landfill that will pump gas out of the ground. In addition, they said, an underground "barrier" will be constructed around parts of Youth Center No. 1 that will prevent the gas from migrating into the facility. They said both systems should be installed in about six months.

Downs said that he was informed by Fairfax County officials last month that the landfill, which is owned by the federal government but operated by Fairfax County, will pay for the protection systems.

Richard A. King, deputy county executive for public safety in Fairfax, said a formal report is due next week that will outline a "comprehensive response" to the leak. He declined to discuss in detail what measures are being considered.

Downs said that inmates may be returned to the facility by the end of the month, and that an alarm system will be installed to monitor the levels of gas inside the facility's buildings. He said the system will automatically trigger a ventilation system when the gas levels are too high.