WASHINGTON - Seven people are confirmed dead and dozens of other injured after two trains collided on the Red line, and investigators still do not know why.

FBI (web) , NTSB, and Metrorail investigators spent Monday night at the scene assessing the damage, retrieving victims, and hoping to determine what went wrong.

Investigators are searching the wreckage for the trains' devices that record operating speeds and commands, NTSB member Debbie Hersman said.

"There is a form of recorders, these data loggers that are on trains. But it really depends on what series of train it is, how old the trains are. Older trains do not have... good recording devices, but some of the newer trains do, so it's possible that we might find some recording devices," Hersman said Tuesday morning.

"The local responders continue some extraordinary efforts performing the recovery operations and that's obviously our first priority, to make sure that all the victims are recovered," Hersman also said. "But our investigators have been working to document the accident scene, taking pictures, doing drawings, making sure that any of the wreckage that might be disturbed during the recovery operations is fully documented so we know exactly where these trains came to rest after the collision."

Homeland Security Department spokeswoman Amy Kudwa said less than two hours after the crash that federal authorities had no indication of any terrorism connection.

It was 5 p.m., the height of the rush hour, and two Red Line trains were headed southbound between the Takoma Park and Fort Totten stations. This particular stretch of the busy Red Line runs above ground.

Metro chief John Catoe said the first train was stopped on the tracks, waiting for another to clear the station ahead, when the trailing train, one of the oldest in the Metro fleet, plowed into it from behind.

Catoe said an automated computer system used to run trains was supposed to keep them apart, but it was not clear whether the system was in use when the crash occurred.

Officials would not say how fast the train was traveling at the time of the accident. The crash occurred in an area with a sizable distance between rail stations in which trains are allowed to travel at higher speeds, Metro spokeswoman Candace Smith said.

Seven people were killed, including the operator of the rear train, Jeanice McMillan of Springfield. Metro spokesman Steve Taubenkibel said McMillan had been a Metro employee since January 2007.

It is the deadliest incident in Metro's 33-year history.

More than 70 people were treated at the scene and six were hospitalized with critical injuries.

Hundreds of emergency personnel responded to the disaster. "Their heroic efforts have been nothing short of absolutely spectacular," said one D.C. Fire and EMS representative.

Eyewitnesses say passengers scrambled for safety and to free themselves from the mangled trains.

"I saw the terror in folks' faces in the back of this train here as they were beating on the windows trying to get out of the cars," recalled one survivor.

No cause has yet been determined, but investigators will be looking into the track signals and operator procedure as possible explanations.

» See ABC 7's photo gallery from the crash here.

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