A fire that killed nine people in a Washington film club for homosexuals last Oct. 24 began when a flammable liquid in the club's carpeting exploded, FBI laboratory analyists have concluded.

What caused the explosion has not been determined, according to the analyst's report conducted for D. C. police.

Flames from the explosion quickly spread to the walls, drapes and stairway of the two-story Cinema Follies club at 37 L St. SE, trapping a group of men who were watching movies. Unable to escape down the burning stairs, or through a blocked upstairs exit eight men died or were dying of smoke inhalation by the time firemen smashed their way into the theater.

The FBI report did littel to resolve a number of mysterious questions surrounding one of the worst fires in the city's history.

"I don't think we'll ever be abale to prove what really happened," one fire department investigator close to the investigation said yesterday.

Homicide detectives at the scene that night said their preliminary conclusion was that fire began when sparks from a rug shampooer touched off cleaning solvent.

The club maintenance man who said he was shampooing the carpet, Martin Crowetz, told reporters at the time that he "heard an explosion and saw fire" under a natural gas heating unit that was suspended from the ceiling near the entrance to the theater.

Crowetz, told reporters shortly after the fire that he and the club manager, Bill Oates, tried unsuccessfully to put out the flames with fire extinguishers.

FBI analysts, who routinely handle laboratory work involving major District fires because of their superior facilities, found traces of two highly flammable liquids benzene and ace-tone, in the carpeting. Each can be used as a cleanig solvent. Acetone also is a substance used in paint remover.

Crowetz, in interviews with reporters immediately after the fire, said he was using "409" and "Lsetoil" shampoo solvent. Neither of these commercial cleaning fluids contain acetone or benzene, the FBI report said.

Crowetz has since refused to appear before investigators to answer more questions, a fire department official said yesterday.

According to informed sources close to the investigation. Crowetz cannot be compelled to appear before a grand jury and answer questions. For that the police and fire department team working the case would have to have evidence of criminal negligence, and they do not have that, the sources said.

The FBI analysts also determined that the upstairs door at the theater was padlocked. Some firemen and at least one customer who survived had claimed the door was padlocked, but Oates and Crowetz and denied it.

Fire investigators concluded that in spite of the locked door, the club appeared to be in compliance with city fire and building regulations at the time of fire. The regulations require that every "assembly room" designed to hold fewer than 74 persons must have one exit door with exit lights above it.

The Cinema Follies met those requirements when it was formally inspected in June, 1976, and in the spring of this year when firemen from a nearby station made an informal inspection, according to fire officials.

In the days after the fire, Chief H. H. Shaffer of the dejpartment's fire prevention division criticized the regulations as "not good enough."

In ruling released yesterday, Corporation Counsel John R. Risher said the fire chief can, "upon finding an imminently dangerous condition," order a place closed within 24 hours.

Other city officials examining fire regulations after the Cineman Follies tragedy pledged to tighten fire safety regulations.

School Supt. Vincent Reed ordered the opening of any exit doors chained or locke, and housing chief Lorenzo Jacobs said his inspectors would "re-interpret" city regulations to require two exits from multi-story theaters such as the Cinama Follies, rather than a single exit as has been required.