Investigators, including the FBI, have begun trying to find out what caused the worst ever crash on Washington’s subway train system.At least nine people died and scores were injured when one commuter train ploughed into the rear of another in a suburb of the US capital. Several victims are said to be in a critical condition. One survivor said: “It wasn’t like we hit something small like somebody was on the tracks. And the people who were on the train, many of them got knocked out of their seat.” Another said:“And the seats flew up. The seats came out the wall and it pushed us, we flew. Everybody on our track flew.” It took emergency crews hours to free some of those trapped inside the wreckage. The accident happened at the beginning of the evening rush hour. But the trains were not full as they were heading into the city, rather than ferrying workers home. The female driver of the train that smashed into the other one was killed. Officials say it may take some time to establish the cause, but that the FBI’s involvement does not mean terrorism is suspected.