Rescue crews and investigators in Washington, D.C., are going through the wreckage of Monday's subway train crash that killed nine people and injured more than 70 others.
Washington Mayor Adrian Fenty told reporters Tuesday that authorities are treating the scene as a rescue operation that is expected to continue all day.
Officials said one train rear-ended another train that had stopped on the tracks near an above-ground train station in northeast Washington.
National Transportation Safety Board (NTSB) spokeswoman Debbie Hersman said the cars on the train that was struck contained data recorders that will be examined once rescue operations are completed.
Hersman said the striking train was made up of older cars that the NTSB had recommended be replaced about three years ago. She said it was impossible to determine what role the older cars played in the developments.
The spokeswoman said the older cars did not contain data recorders, but the tracks and other physical evidence will be examined to determine what happened moments before the crash.
The impact was so hard the lead car of the moving train was sliced open, vaulted into the air and landed on top of the rear car of the other train. A crane was brought in early Tuesday to help separate the wreckage. The rescue crews are working with the NTSB to gather evidence related to the accident.
Hersman said cell phone and text records also are being gathered to see what can be learned from them about the crash.
The female operator of the moving train is among the dead. Officials said at least six of the injured passengers are in critical condition. Mayor Fenty called the crash the worst in the rail system's 33-year history.
U.S. President Barack Obama and his wife, Michelle, issued a statement Monday night expressing their condolences to the victims.
Some information for this report was provided by AP.