An Air Florida jetliner carrying at least 76 persons crashed into the northbound span of the 14th Street Bridge and plunged into the icy Potomac River in the midst of a blinding snowstorm yesterday afternoon.

There was no official count last night of the number of passengers and motorists killed. Local hospitals reported treating only five survivors from the plane. At least 71 were unaccounted for and believed to be dead.

The twin-engine Tampa-bound Boeing 737, flight number 90, had just taken off from Washington National Airport at about 4 p.m. when it appeared to settle down near the bridge, according to several witnesses. The plane's wheels, which evidently had not yet been retracted, reportedly struck five cars, crushing some of the occupants. Witnesses reported seeing two cars knocked from the bridge into the river.

The cause of the accident was not immediately known. A spokesman for the Federal Aviation Administration said there was no indication that the air traffic controllers' strike had anything to do with the crash. The airport was closed at 4:28 p.m. and did not resume flights until 8 p.m. During that period, traffic was diverted to Dulles.

Using helicopters, which hovered dramatically close to the Potomac and at times dipped into it, rescue workers pulled survivors clinging to the plane's wreckage from the ice-filled river. Rescue efforts were hampered both by the large chunks of ice and by a mammoth traffic jam caused by homebound commuters who had left work early. When one surviving passenger was unable to grasp the lifeline dangling from a rescue helicopter, a man dived into the icy water, swam to the helpless victim and hauled her to safety.

One passenger, identified as Joe Stiley of Alexandria, described his ordeal. "Once we got started," Stiley said, "it seemed like the pilot was trying to abort it. I went unconscious. I think the water revived me.

"The biggest problem was getting off the seatbelt. I saw a helicopter. I saw three gals in the water. I was trying to hold the two gals and me but there were ice chunks. They were a problem. I couldn't hold on to anyone. Then one guy on the bank tried to swim out with a rope. A lot of people were trying to throw ropes to us from the bridge but we couldn't move. Our only chance was to hold on. Everybody there had broken bones. My legs are both crunched."

Late last night, Robert Silverberg, Air Florida's vice president for legal affairs, said the plane had carried 71 ticketed passengers, five crew members, and possibly three infants.

Survivors were taken to three hospitals: George Washington University Hospital, the Washington Hospital Center and the National Orthopaedic & Rehabilitation Hospital in Arlington.

The accident was National Airport's first fatal commercial crash since Nov. 1, 1949, when a twin-engine P38 Lightning fighter plane plowed into the rear of a four-engine Eastern Airlines DC4 that was landing. All 51 passengers and the crew of four on the airliner were killed.

Yesterday's crash also was the first commercial disaster involving an American jet in the United States in 26 months. The last crash of an American commercial jet occurred on Oct. 31, 1979, when a Western Airlines DC10 landed on a wrong runway in Mexico City and struck a truck. Seventy people died in that accident.

Visibility at the time the plane took off yesterday was only half a mile, according to the National Weather Service. Controllers in the tower cab on top of National's main terminal could not see the north end of the runway. It is up to the pilot, based on procedures developed by his airline and approved by the Federal Aviation Adminstration, to decide whether to fly in bad weather.

According to sources, the plane was radioed the instruction, "Cleared for takeoff," a standard command that orders the pilot to roll his plane. No acknowledgement is required of the pilot, and officials said they will have to replay the air traffic control radio tapes before they know if the pilot said anything in response.

The plane immediately started rolling north on Runway 36, National's longest at 6,870 feet.

The air traffic controllers in the tower cab were unable to see the plane leave the ground. During weather conditions such as yesterday's, controllers monitor flights on a radar screen mounted above their heads. If all had gone normally, shortly after the plane disappeared from view a blip should have appeared on the radar, accompanied by computer-generated data giving the flight number, altitude and other information.

That blip, if it appeared at all, was there only for two sweeps of the radar beacon -- a total of eight seconds, according to the sources. The plane never appeared on the larger, more sophisticated, easier-to-read radar in the darkened instrument room immediately below the tower cab.

The tower received no distress calls from Air Florida Flight 90 before it crashed.

When the larger radar screen failed to register the plane, air traffic control officials scrambled. They had to assume, in the absence of a radar track, that the plane was down.

Until an official version of the crash is given, the best initial account available is based on reports from witnesses.

Michael Paullaitis, an aviation technician with the Washington-based Coast Guard air station, said that he and members of his unit based at National Airport were delayed from taking off on their own flight because of the weather.

"We saw this plane taking off," Paullaitis said. "It was low and it looked like it was having a hard time."

Another witness, John Noble, a psychologist at Lorton, was on I-395 going north toward Washington. "I saw the plane coming toward the bridge," Noble said. "It either didn't have enough power or something went wrong. The tail came down and hit the bridge and burst into flames. You could smell fuel and it hit the water and fell apart."

Marcus Stein, an Alexandria accountant, said that the "plane came over the bridge and sort of slid on top of the cars and took a few over with it. I couldn't see how many. There was a lot of fire. The plane went into the water after that."

Air Force Sgt. Jerome Lancaster, who was stuck in the traffic jam on the bridge at the time, said that one of the plane's wheels hit a truck before skidding into the water. An Associated Press photographer, Ron Edmonds, said he saw people strapped into airline seats under the water.

Another witness, Terrence Bell, said the plane hit five or six cars and a truck on the bridge, shearing off the roofs of some of the vehicles. Rescuers tried to free two bodies in the crushed vehicles, but were stymied by the fact that the tops of the cars had been flattened to door level.

Several Park Police and Coast Guard helicopters arrived soon after the crash, some dropping ropes with flotation rings to survivors floundering in the river, others carrying at least one survivor to a nearby hospital. The helicopters hovered close enough to the river's surface to pull some people out of the Potomac by their hands and wrists.

D.C. Fire Department officials dispatched five ambulances to the scene, but they were slowed by the snow-bound traffic and several ambulances from Arlington arrived at the bridge first. Backup ambulances also were sent by Prince George's County.

Eight survivors were rushed to the three hospitals. Five were pulled from the plane, according to hospital officials. The other three had been in vehicles on the bridge.

Patricia Felch, 27, of Fairfax, who recalled being thrown from the plane into the water, arrived by helicopter at Washington Hospital Center's Medstar unit at about 5:05 p.m. She was in serious but stable condition after undergoing surgery for multiple fractures and a lung contusion, a hospital spokeswoman said.

Five survivors arrived by ambulance at Arlington's National Orthopaedic & Rehabilitation Hospital at 5:05 p.m., a hospital spokeswoman said. Kelly Duncan, a stewardess from Miami, was in stable condition. Priscilla Tirado, an area resident, was in critical condition, suffering from shock, the spokeswoman said. She said that the remaining three were suffering from trauma and other less severe injuries.

Two male survivors listed in serious condition with multiple injuries were taken by ambulance to George Washington University Hospital. The first man, who arrived at 4:45 p.m., had his head bandaged and appeared unconscious and was taken to the intensive care unit, a hospital spokeswoman said.

Lenny Skutnik, 28, of Lorton, Va., said he dived into the river when a woman lost her grip on a helicopter line and fell back into the water. He said he dragged her to shore. He said she cried out for help and then passed out. Skutnik carried her to a waiting ambulance, where she was taken to the National Orthopaedic & Rehabilitation Hospital.

Other rescuers in boats used steel poles to probe the broken ice, which was spotted with blood. "I don't think we're going to get any more out alive," said Arlington emergency services coordinator James Trollinger.

About three dozen relatives and friends of passengers huddled in a room at the Crystal City Marriott, awaiting some definite word. Some were crying; others sat silent.

"It's the uncertainty that's getting them," said Father Charles Perloff, one of three clergymen who tried to comfort those who were waiting. Perloff said that one 24-year-old man at the hotel was waiting to hear about his wife and two children, who had taken the flight to visit the family's cancer-stricken grandmother in Florida. Another man said that his mother, 84, was on the plane.

Throughout the late afternoon and evening, dozens of divers had searched for bodies in the river. But at 9:18 p.m., the D.C. Harbor Patrol called off the search for the night.

The divers had recovered the badly battered bodies of three men and two women, which police officials said would be examined today at a temporary morgue on the Southwest waterfront.

Maj. Harold Anderson, the rescue official in charge, said his divers assumed that the remaining passengers were strapped in their plane seats.

After the rescue and recovery efforts were called off, a spokesman for the National Transportation Safety Board said that barges would be brought in today to assist in recovering the aircraft. He said the first priority was to retrieve the plane's recorders.

A Coast Guard official said the damaged plane might be stored at the Coast Guard air station at National Airport for investigative purposes. He said a team of underwater specialists has arrived from Elizabeth City, N.C. to help recover the hulk of the plane.

The National Transportation Safety Board, which is charged with determining probable cause of accidents, set up headquarters at Hangar Six at National Airport last night. Board experts will study technical data and air traffic control procedures and will interview scores of witnesses.

One witness said that the plane's landing gear appeared to be extended at the time the plane hit the bridge. That would be highly unusual, according to pilots and other aviation sources. Landing gear creates enormous additional aerodynamic drag for an airplane, thus requiring more power to keep the plane airborne.

Experts, cautioning against early speculation, listed three possible reasons for the crash:

Loss of power. A failure in one of the two engines could be serious under certain circumstances, although two-engine aircraft such as the 737 are required to demonstrate that they can fly successfully with one engine, especially at critical moments such as takeoff. A two-engine failure automatically would be catastrophic because jetliners do not glide without power.

Ice on the control surfaces of the wings. Ice adds additional weight to the plane and makes operation of the vital control surfaces sluggish or even impossible in extreme cases. This potential cause clearly is a prime suspect because of the weather at the time of the crash. It is standard practice in such weather to spray wings and control surfaces with de-icer immediately prior to takeoff. There are accidents on record, including a recent one in Clarksburg, W.Va., where a plane, after spraying, sat around for awhile before takeoff. By the time flight was attempted, the wings had iced again. The plane crashed at the end of the runway.

Loss of vital instruments, including the gyro, which gives a pilot his "up and down" reference. There were reports that the plane had lifted off in a snow squall. In such situations, the pilot would have to rely solely on his instruments instead of on visual cues he might get out the cockpit window. In the extremely rare circumstance where he might lose his "up and down" reference, he would literally be flying blind.