Human error -- Possible mistakes by a subway supervisor or operator--was cited yesterday by federal investigators as a key factor in Wednesday's violent derailment in a downtown Metro tunnel, the first fatal wreck since the subway system opened six years ago.

"There was clearly some kind of breakdown in the human performance in the switching," said Patricia Goldman, the National Transportation Safety Board member overseeing the U.S. probe of the accident, which left three passengers dead and more than 20 injured. She referred to an improperly aligned rail switch that, officials say, led to the wreck. The switch was being manually operated.

"It would be human error in manually setting the switch," said Harold Storey, chief of the federal safety agency's railroad accident division. Goldman and Storey both cited additional issues centering on possible human error that are being investigated.

At a news conference, Metro General Manager Richard S. Page promised that Metro would "redouble" its safety efforts. "All I can say to the public is that we have taken every precaution that we know how," Page said. "Our first obligation is to assure the safety of the Metro subway system."

The three passengers killed in the accident aboard the crowded, rush-hour train were identified yesterday as Mariano Cortez, 41, of Riverdale; Mildred M. Morgan, 71, of Hillcrest Heights, and Mary L. O'Meara, 25, of New Carrollton. Officials said they were killed by the impact caused by the derailment.

The accident occurred as the six-car Orange Line train bound for New Carrollton approached the Smithsonian station, 12th Street and Independence Avenue SW. According to Metro officials, the subway, with an estimated 1,200 to 1,500 homeward-bound passengers, reached a rail switch about 650 feet from the station. The switch should have been manually adjusted to allow the train to continue traveling on the southbound track.

Instead, officials said, the switch was improperly aligned and the train started to traverse the crossover and enter the northbound track--the incorrect track. The operator stopped the train after the first car had begun to cross. A Metro supervisor, who was in the tunnel to adjust the switch, boarded the train's rear car and took over the controls, officials said. The train backed up, apparently in an attempt to get back to the correct track. Immediately the derailment occurred.

The front wheels of the car that had entered the incorrect track remained on that track. With its front and rear wheel on separate tracks, the rail car was dragged diagonally through the tunnel for a short distance until it was crushed against a large concrete divider situated between the two tracks to help prop up the tunnel's roof. Most of the victims--and all three of those who were killed--were in this car, the only heavily damaged part of the train.

In an interview, Joe H. Sheard, Metro's rail services director, said the rail switch at the accident site had been operated manually for only 10 to 15 minutes at the time of the incident. Normally, switches are electronically activated, but they may be adjusted manually when electronic equipment malfunctions, as it did Wednesday. Safety officials say manual switching is an acceptable practice, provided that proper procedures are followed. Officials, Sheard said, have not yet determined why the electronic switching equipment failed and manual switching had to be substituted.

All three passengers who died on the train were seated on the left side of the damaged car--the side that was driven into the concrete divider. Witnesses said they appeared to be killed almost immediately.

O'Meara had been traveling home from work, her father said yesterday. Thomas O'Meara said his daughter had passed the Maryland bar examinations in July, and later in the year was admitted to practice law. She was employed by the National Education Association in the District, he said, and was planning to get married this summer. "She had everything ahead of her," he said.

Cortez was identified as an accountant with the U.S. Department of Agriculture. Friends or relatives of the third victim, Morgan, could not be reached yesterday.

Witnesses painted a harrowing scene. One of them, 28-year-old Ruth Cannon of Capitol Heights, said she was standing in the crowded aisle beside the three victims when they died.

Two of the victims were seated next to each other on a seat on the left side, in the rear half of the car, she said. The third victim was seated just in front beside an elderly man, on a seat facing the aisle.

"When we hit, everything just crashed in as though the tunnel had caved in, Cannon said. "We hit so hard. We weren't going that fast . . . . All I saw was glass on top of me. The side of the train was pushed upwards over me. I was taller than the people standing around me; I was just amazed."

Immediately after the side of the train was pushed in, Cannon said, "the floor split open and came away from under us. A couple of people standing beside me fell into the hole . . . then I went down the hole." She said several people fell on top of her. As the passengers struggled to extricate themselves from the tangle of bodies, Cannon said, she looked at the people who had died beside her.

Josh Bowers, a private investigator who lives on Capital Hill, said nobody in the third car, where he was riding, realized how serious the accident was. "There was no screaming or anything," he said. "It seemed odd. For some reason the word didn't filter back."

Several minutes later, however, panic struck when a fireman walked past the car wearing a gas mask and carrying an air tank, he said. Passengers tried to open the doors but couldn't, he said because the emergency instructions on the door had faded to illegibility.

An unidentified doctor who was on the front car was able to check injured patients, passenger Cannon said. But even when emergency workers arrived, she said, they said they lacked some emergency medical equipment, such as splints, which had been taken to the scene of Wednesday's disastrous airplane crash near the 14th Street bridge.

"One guy's leg was broken," she said. "We didn't have any splint. We eventually found a pieece of padded board." She said they also struggled to lift another man, who said he weighed 250 pounds, who had a broken leg.

Cannon said it was about 30 to 45 minutes before those who were uninjured were able to leave. She said she was the last to leave, because she stayed with an injured friend, Deborah Davis of New Carrolton. They had met each other, for the first time in almost 10 years, at the Farragut West Station before boarding the train.

Yesterday, Deboarah Davis was listed in good condition at Washington Hospital Center, with a broken right leg and possible abdominal injuries.

At the accident site early yesterday, the wrecked rail car could be seen wedged diagonally across the tunnel's width. A 15-foot segment of its wall was severely twisted. Part of its floor had caved in. Seats were crushed together. Windows were bent and cracked. Much of the roof had been smashed.

A second investigative panel began to assemble in Washington yesterday and is scheduled to meet at 1:30 p.m. today with Metro's board of directors in a public session. The investigative board, sponsored jointly by Metro and the American Public Transit Association, is to conduct an inquiry separate from that of the National Transportaion Safety Board and make recommendations. Its chairman is Frank Gorman, general manager of the Port Authority Trans-Hudson system of New York.

Metro officials declined to identify the subway operator and supervisor involved in Wednesday's derailment. "First of all, they're being questioned. Second of all, we don't want to hang them," explained Metro spokesman Cody Pfanstiehl.

While mechanical or electrical failures may have figured in the accident, federal safety officials made clear yesterday that much of their investigation would focus on possible human failure. Transportation safety board member Goldman said that key issues include whether Metro employes were properly trained for manual switching operations, whether proper procedures had been set up and followed, and whether the evacuation of stranded passengers was properly carried out. Some passengers have complained of a frightning wait in darkness before rescue workers reached them.

Storey, the safety board's railroad accident chief, cited other possible issues involving human error. Investigators must determine, he said, whether the supervisor properly readjusted the manually operated switch before boarding the train to back it up. "You never back up through such a switch before inspecting the switch itself ; you've got to get off the train and hand-crank it in place," he said.

Other questions, Storey suggested, are whether the train operator properly obeyed signals when he entered the switch and whether Metro control room officials gave proper commands to the train operator.