Three passengers were killed and at least 22 others injured yesterday when a crowded Metro subway train derailed at rush hour near the Federal Triangle station in downtown Washington.

The accident occurred on a New Carrollton-bound Orange line train, and severely disrupted afternoon and evening service on the Blue and Orange lines. About 1,200 persons had to be evacuated from the system, officials said. The derailment was the first fatal accident on Metrorail since the system opened in 1976.

At a news conference late last night, Metro general manager Richard S. Page said a special panel, including transit officials from New York, Chicago and California, had been set up to investigate the accident. "We don't know the cause at this point," he said. "We're still looking for facts."

The derailment occurred, Metro officials said, while the train was running at a slow speed under manual, rather than computerized, control. Joe H. Sheard, Metro's rail services director, attributed the accident partly a malfunctioning rail switch. The switch, he said, shifted the subway to an incorrect track. When the train backed up to get on the proprer track, he added, its front wheels remained on the wrong track and the train was derailed.

D.C. Assistant Fire Chief Theodore Coleman, who arrived at the scene shortly after the accident, said, "The front end of the train was just crushed."

Two hours after the 4:30 p.m. derailment, injured passengers were still being carried from the station on stretchers.

The accident, which apparently occurred as the train backed up across a switch in the tracks, left pieces of the rail car strewn about the tunnel and several passengers caught in the wreckage. "I saw a woman trapped between the train and the wall. . . . She looked like she was dead," one passenger said.

The victims were initially described as a man, about 60 years old, and two women, one in her 20s and the other somewhat older. Their identities were not immediately made known.

Officials said 15 of the injured were taken to Washington Hospital Center, four were taken to Howard University Hospital and three to George Washington University Hospital. Most were said to be suffering from neck or back injuries. One of the injured at Howard Hospital was in serious condition.

Emergency workers used a powerful, hydraulically operated saw to cut through the side of the subway car to remove the victims' bodies. Firefighters and rescue workers were called in from Montgomery and Prince George's counties and from Bolling Air Force Base in far Southwest Washinngton to aid in the evacuation.

The accident occurred shortly after the Air Florida plane crash near the 14th Street Bridge, and combined with the air disaster and yesterday's snowstorm to create a massive commuter snarl and severely tax the city's emergency services.

Harold Storey, chief of the railroad accident division of the National Transportation Safety Board, who was at the scene, attributed the accident partly to a failure of the train's wheels to cross a rail switch. "On the last car of the train, the wheels continued straight instead of making the crossover," he said. The switching mechanism was designed to allow trains to move from one track to another.

Witnesses said the train stopped shortly after pulling out of the Federal Triangle station, located under 12th Street NW between Pennsylvania and Constitution avenues, then began to back up.

"I heard the conductor say over the intercom to someone, 'Can I see myself clear to back up,' " a passenger recalled. "He started backing up the train and then it jammed into the wall. The whole train jolted and the people started falling all over each other. Everybody who was standing was falling. People started hollering and panicking and the lights were out."

The violent derailment "collapsed one end of the car. The roof and side just caved in, I assume from hitting the concrete tunnel side," recalled J. Philip Hinson, a Treasury Department economist who also was aboard the train. Jeanne Wilson, another passenger, said, "You felt the impact pretty hard. You didn't fall down because it was so crowded."

Arthur Hastings, a Bowie businessman aboard the subway, said, "The center wall of the tunnel gradually crunched right into the train like a slow-motion movie. Glass was flying. People were screaming. My wife dropped down through a hole. Her legs were caught." She escaped about 20 minutes later with a broken leg, Hastings said.

After the derailment, passengers described a harrowing time, perhaps a half-hour, as they waited in darkness for help.

"There was a cable that was cut above the train. It kept discharging in flashes and pops," recalled Aurora M. Gallagher, a staff member of the National Academy for Sciences. Two passengers seated side by side near her appeared to be dead, she said. Another was crying out for help, shouting, "Demon, Demon, I can't stand it. Please get it off of me."

The derailment immediately halted all subway service between McPherson Square and the Federal Center stop, at Third and D streets SW, and led to delays on other parts of the Blue and Orange lines. At the same time, Metro officials reported some minor delays on the Red Line, though they said these did not stem from the accident.

As the disruption grew, homeward-bound passengers lined up outside other subway stations in the snow, grumbling and confused. Eventually, the Metro system established special bus service to transport riders along the shutdown section of the Blue and Orange lines. Transit officials also extended regular rush-hour bus service into the evening to help stranded passengers get home.