At the time, both trains were on the same track, and one of them was stationary when the crash happened, said John Catoe, the Metro Authority's general manager.

The impact was so severe that rescuers had to cut through the trains to get people out from the wreckage. The Metro Red Line has ceased operation for at least Monday night.

Officials suspect there may be other corpses trapped inside the wrecks so the final death tally will probably grow.

But they said they do not know the cause of the collision yet and are not likely to know the cause for days as the investigation unfolds.

The National Transportation Safety Board said it will take part in the investigation and FBI agents were at the scene Preliminary investigations by the Department of Homeland Security show there was no indication of anything other than an accidental collision, though more details need to be sorted out.

It was the second Metro crash that involves fatalities in the 33-year history of the transit authority.

A police officer guides a woman who wants to make sure if there is her relative on the train, at the site where two subway trains collided on the Red line between the Fort Totten and Tacoma stations in the northeastern part of Washington D.C., on June 22, 2009. [Zhang Yan/Xinhua]

In January 1982, a derailment killed three people. The only other collision between Metro trains occurred in 2004.

"We are extremely saddened that there are fatalities as a result of this accident, which has touched our Metro family," the Metro Authority said in a statement.

"Our safety officials are investigating, and will continue to investigate until we determine why this happened and what must be done to ensure it never happens again."