An 18-year-old woman who said she was held against her will for longer than five hours in a dormitory at the Lorton Reformatory, has told the FBI that she was raped by at least a dozen inmates in the minimum-security section after she had attended a disco dance there Saturday night.

An FBI spokesman said yesterday that agents are investigating the woman's allegations, as well as the question of how her presence in the District reformatory for men went undetected for that long a time before guards found her hidden in a bed.

A total of 40 visitors -- 35 adults and five children -- were admitted to the reformatory for the dance program and were supposed to leave the facility when the event ended at 10 p.m. Saturday, according to D.C. Department of Corrections officials. But it wasn't until 3:15 a.m. Sunday that guards, acting on a tip from an unidentified inmate, discovered that the woman had remained in the reformatory, corrections officials said. She had gone to the D.C. Corrections Department facility in Fairfax County to visit a boyfriend, investigators said.

Samuel Rosser, who is in charge of the minimum-security unit, said visitors there -- unlike those to the more restricted sections of Lorton -- are not required to sign in or out. Instead, Rosser said, guards admit visitors if their names appear among those supplied by inmates.

When the guards made a "head count" of the visitors as they left Saturday's dance, Rosser said, they did not notice they were short by one.

The FBI spokesman said that when an agent interviewed the woman at D.C. General Hospital, where corrections officials took her for treatment, the woman told the agent she was forced to have sex with 12 inmates after they held her in the dormitory against her will. The spokesman said that the woman told authorities that she was afraid to press charges and would not testify as a complainant in any criminal case.

The investigation, which is being conducted by the FBI's Northern Virginia office, was still under way yesterday. Its results will be presented to the U.S. attorney's office in Alexandria for a decision about whether to bring charges, the FBI spokesman said. He said a preliminary examination at the hospital indicated that the woman had had sexual relations.

Rosser said when he questioned the woman a short time after she was found, "she never said she was raped." She did say, Rosser added, that she had had relations with her boyfriend and several other prisoners.

Rosser said other prison sources have said that the woman had agreed to have sex with two or three inmates as well as her boyfriend. But, Rosser said, the situation "may have gotten out of hand" and other inmates may have taken part.

Kenneth Bynum, president of the union that represents D.C. corrections guards, said the incident illustrates that Lorton "is run . . . loosely."

"It's wide open, we have no control," Bynum said. Sex between inmates and visitors and drug usage is common at Lorton, Bynum said. A law enforcement source said the FBI has also learned that there have been several incidents at Lorton in which inmates had urged girlfriends to have sex with other inmates for money.

The minimum-security prison houses about 190 men. It has no walls or fences.