(Reuters) - The investigation into smoke in a Washington subway tunnel that killed a woman and sent 84 people to hospitals could take 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 metres) ahead of the train, NTSB spokesman Peter Knudson said.
"What caused the electrical arcing is what we want to understand," he said. Arcing happens when electricity from the power rail comes into contact with something that conducts electricity, like water.
The smoke occurred about 800 feet (240 metres) 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.
Emergency response will be among the areas investigated, the statement said. The Washington Post quoted some passengers as saying it had taken firefighters an hour to reach the train.
Eighty-four people were taken for treatment to three Washington hospitals and two were critically injured. The condition of one of those listed as critical was upgraded to serious, a spokeswoman for MedStar Washington Hospital Center said.
In a statement, Tom Downs, the chairman of the Washington Metropolitan Area Transit Authority, apologized for the incident. He said that once the cause was determined, Metro was prepared to keep it from happening again.
The smoke near the L'Enfant Plaza station, a key transfer point for five Metro lines, also caused major disruptions on other lines.
Metro was running with delays on five of six lines on Tuesday.
The system, which operates in Washington as well as the Virginia and Maryland suburbs, is the second busiest nationally in terms of passengers carried, after New York's subway network.
The incident on Monday was the first on the Metro system involving a passenger death since a crash in June 2009 that killed eight riders and a train operator.
(Reporting by Ian Simpson; Editing by G Crosse and Peter Cooney)
