WASHINGTON - The investigation into Washington subway smoke that killed a woman and sent 84 people to local hospitals could take from six to 12 months, a spokesman for the National Transportation Safety Board said on Tuesday.

The preliminary cause of the smoke that filled six subway cars in a tunnel on Monday was electrical arcing involving the charged third rail about 1,100 feet (335 meters) ahead of the train, NTSB's spokesman Peter Knudson said.

"What caused the electrical arcing is what we want to understand," he said.

The smoke occurred about 800 feet south of the Washington Metro's L'Enfant Plaza station in a tunnel leading to the Pentagon Metro station. Passengers evacuated themselves from the stalled cars and there was no fire on the train or derailment, the NTSB said in a statement.

Eighty-four people were taken for treatment to three Washington hospitals, and at least two people were critically injured, officials said. The hospitals had no immediate word on their conditions.

The smoke near the L'Enfant Plaza station, a key transfer point between multiple Metro lines, also caused major disruptions on other lines.

A spokeswoman for the Washington Metropolitan Area Transit Authority said the system was operating as planned on Tuesday.

"We had no reports of crowding or any operational issues," she said.

The system, which operates in Washington, D.C. as well as the Virginia and Maryland suburbs, is the second-busiest nationally in terms of passengers carried after New York's subway network.