Two Washington DC, subway trains collided during the Monday afternoon rush hour, killing nine people and injuring at least 75, officials said early on Tuesday.

Mayor Adrian Fenty called the crash the deadliest in the 33-year history of the city’s Metro subway system. Officials said one train hit another train that had stopped at a platform, but the cause of the crash was not immediately clear. At least one car from the trailing train was hurled onto the top of the other in the accident, which occurred on above-ground tracks.

“Metro officials do not know the cause of the ... collision and are not likely to know the cause for several days as the investigation unfolds,” the Washington Metropolitan Area Transit Authority said on its website.

The crash occurred on the heavily travelled red line about 5 pm EDT (2100 GMT), between Fort Totten and Takoma stations on the northeastern outskirts of the city near the border with Maryland. Both trains were heading south into the city.

It was the first crash involving a passenger death since 1982, when three people were killed in a derailment. The Metro train system began service in 1976.

“What happened ... (was) one train was stopped waiting to get the order to pass. ... The next train came up behind it and, for reasons that we do not know, collided into the back of that train,” John Catoe, general manager of the Washington Metropolitan Area Transit Authority, said.

“We are committed to investigate this accident until we determine why this happened and what must be done to ensure it never happens again,” Catoe said in a statement.

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