A young Fairfax County woman was rescued from her car yesterday after it plunged into the Anacostia River and she was trapped under water for about 45 minutes, D.C. police reported.

Police said Mary Jane Cross, who is in her 20s, of 780, Belford Dr., was seen by witnesses about 12:55 p.m. at the foot of the 11th Street Bridge, parked off Anacostia Drive. Witnesses said they saw her car began to move, then stop, and then accelerate sharply and plunge into the river.

A civilian Army Crops of Engineers employee, John Coley, 46, said he saw the car go into the river from a barge he was working on, and made a futile attempt to reach the woman in a rowboat.

An unidentified passerby flagged down a U.S. Park Police officer who called D.C. harbor police and an ambulance as well as police and Navy divers, police said.

The divers pulled Cross out of the car about 1:40 p.m. Police theorized that an air bubble in the vehicle allowed her to stay alive under water that long.

Ambulance technicians administered artificial respiration and gave Cross oxygen. She was taken to D.C. General Hospital, where she was placed on a mechanical respirator.

Police said Cross is a widow with two children who had been under doctor's care since the death of her husband four years ago.