A young woman was killed when she fell onto the subway tracks and was struck by a train on the Upper East Side on Wednesday afternoon, possibly after fainting, law enforcement officials said.

The police said that no criminality was suspected, although an investigation was continuing.

The woman was identified by the police as Fatoumata Diallo, 21, of the Bronx.

The death occurred at East 77th Street and Lexington Avenue, along the No. 6 train line, officials said. The first 911 call was received at 3:09 p.m., said Frank Dwyer, a spokesman for the Fire Department. The train that hit the woman was a rerouted No. 5 train making local stops along the No. 6 line, said Kevin Ortiz, a spokesman for New York City Transit. The train’s operator was interviewed by investigators, Mr. Ortiz said.

Service was disrupted afterward. In both directions, No. 6 trains ran express between 125th Street and Grand Central Terminal, and some downtown No. 6 trains ended at East 86th Street, Mr. Ortiz said. By 7 p.m., service had returned to normal, he said.