Seeping methane gas triggered the second explosion in four days in a dormitory complex at the Lorton Reformatory yesterday, critically burning an inmate and forcing the evacuation of nearly 400 prisoners.

The gas seeped into 11 other buildings at Youth Center No. 1 at the prison complex, operated by the District but located in Fairfax County, apparently through sewer lines and telephone cables from a nearby 300-acre landfill, authorities said.

"We checked and found gas in almost all the buildings," said Leroy Anderson, spokesman for the D.C. Department of Corrections. "We're going to determine the source and how to shut it off . . . . No one has told me he or she is afraid of additional explosions."

The prisoners were taken by buses to other detention facilties on the huge reformatory grounds in southern Fairfax County, while two police helicopters hovered and heavily armed police stood ready.

Anthony Johnson, 25, was injured in yesterday's incident about 5 a.m. and was listed in critical condition at Washington Hospital Center with second and third degree burns over 86 percent of his body. Arthur Moody, 26, who was burned in the first blast about 5:30 a.m. Monday and was listed yesterday in serious condition at the hospital with second and third degree burns over 36 percent of his body.

The two stayed in the same dormitory, about six rooms apart, and both reportedly were smoking cigarettes when the explosions occurred. Johnson had been serving a six-year prison term for burglary and car theft; Moody was serving a six-year sentence for grand larceny and car theft.

Anderson said corrections officials had been unable to determine the cause of the first flash fire on Monday but had suspected it might have involved a cigarette igniting an aerosol spray or cleaning fluid. Another theory was that the inmate had been "free-basing" cocaine when the fire erupted.

An official of the union local that represents D.C. correctional officers sharply criticized prison officials yesterday, accusing them of failing to investigate the first fire adequately and evacuate the area. He called for a thorough independent investigation of the fires and conditions at the Youth Center and surrounding facilities.

"The department has been very negligent in dealing with this problem," said Hugh (Rusty) Hassan, a trustee of Local 1550 of the American Federation of Government Employees (AFGE).

Asked late yesterday whether he as completely satisfied with Monday's investigation, James Palmer, director of the department, replied: "I don't know yet."

The men's dormitory is part of a cluster of residential, vocational and administrative buildings just inside the reformatory's front gate and about 700 feet west of the large landfill, which has been in operation since 1972.

The landfill, which handles nearly 4,000 tons of refuse daily, is used by Fairfax County, Arlington, Alexandria and the District. Experts say that landfills often generate large quantities of methane gas, produced through the decomposition of organic material.

The dormitory where the explosions occurred is heated by oil, and investigators yesterday ruled out the possibility that the gas could have come from inside the building.

Tom Julia, a spokesman for Washington Gas Light Co., which sent a crew to the scene, said the methane gas probably seeped into the dormitory over a period of time and was lit by sparks, possibly from cigarettes, that caused the flash explosions.

The 52 other inmates in the dormitory were evacuated immediately after the second explosion and taken to a gymnasium. Fairfax County fire and rescue units arrived at the prison and later in the morning inmates in a nearby vocational building also were evacuated as a safety precaution.

A spokesman for the AFGE local said the union demanded that the entire complex be cleared of inmates and correctional officers about 11:30 a.m., after it became clear that methane gas had escaped into the buildings and manholes.

"There was a rumor floating around that we were going to walk out if they didn't evacuate," the spokesman said. " . . . The word methane just set everybody off."

Palmer ordered the evacuation about 2 p.m. and prisoners began moving out about 3:20 p.m. By evening, 12 busloads of prisoners -- many of them yelling out the windows to reporters -- had been moved to other buildings within the prison grounds.

Fairfax County sent in 25 officers and two helicopter crews and the D.C. police department's special operations division assigned about 30 officers to help oversee the caravan of prisoners. The police were heavily armed, carrying shotguns and rifles. About a dozen Fairfax fire department vehicles were stationed outside the main gate to be ready in case there were additional explosions.

The evacuation caused intense overcrowding in other parts of the already crowded prison complex and some prisoners were bedded down on the floor. According to officials, 122 of the prisoners were placed in the Occoquan Detention Facility, 82 went to the central holding facility, 190 went to Youth Center No. 2 and its gymnasium and five went to maximum security.

Prisoners went through a "shakedown" before being transferred. Corrections officials said they probably will go through the facility with a "fine-toothed comb" to check for drugs and other contraband before returning any inmates, according to a spokesman. Prison officials were uncertain how long it would be before inmates could begin returning.

Lorton reformatory, the city's penitentiary with a capacity of 2,809, is 20 miles southwest of the D.C. line. The six-prison complex spreads over 3,000 acres of rolling hillsides.

The prison, which has long been plagued by security problems and drug use, has been a frequent target of criticism by Virginia officials.

Last September, a stinging report commissioned by the Fairfax County government said the Lorton prison complex was threatened by major security problems that left the facility "vulnerable to attack and takeover" by inmates who often appear to be "running the place."

About 200 maximum security inmates staged a nine-day work stoppage in July over visiting conditions and the end of "family religious day visits." Just before a settlement was reached, five inmate religious leaders of the work stoppage were transferred out of Lorton to the D.C. Jail to await transfers to federal prisons.