WASHINGTON - One Metro transit train smashed into the rear of another at the height of the capital city's Monday evening rush hour, killing at least six people and injuring scores as the front end of the trailing train jackknifed violently into the air and fell atop the first.

Cars of both trains were ripped open and smashed together in the worst accident in the Metrorail system's 33-year history.

District of Columbia fire spokesman Alan Etter said crews had to cut some people out of what he described as a "mass casualty event." Rescue workers propped steel ladders up to the upper train cars to help survivors scramble to safety. Seats from the smashed cars spilled onto the track.

D.C. Mayor Adrian Fenty said six were confirmed dead. Fire Chief Dennis Rubin said rescue workers treated 76 people at the scene and sent some to hospitals, six with critical injuries. A search for other victims continued into the night.

A Metro official said the dead included the operator of the trailing train. Her name was not immediately released.

The crash about 5 p.m. took place on the system's red line, Metro's busiest, which runs below ground for much of its length but is at ground level at the accident site near the Maryland border in northeast Washington.

Metro chief John Catoe said the first train had been 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.

Officials had no explanation why the second train had not stopped. The National Transportation Safety Board took charge of the investigation and sent a team to the site.

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. The trains' devices that record operating speeds and commands are being turned over to the NTSB, Smith said.

Each train had six cars and was capable of holding as many as 1,200 people. Safety Board member Debbie Hersman said the trains had been bound for downtown. That would mean they were less likely to be filled during the afternoon rush hour.

More than 200 firefighters from D.C., Maryland and Virginia converged on the scene. Sabrina Webber, who lives in the neighborhood, said the first rescuers to arrive had to use the "jaws of life" to pry open a wire fence along the rail line to get to the train. Webber raced to the scene after hearing a loud boom like a "thunder crash" and then sirens. She said there was no panic among the survivors.

Passenger Jodie Wickett, a nurse, told CNN that she was seated on one train, sending text messages on her phone, when she felt the impact. She said she sent a message to someone that it felt as if the train had hit a bump. "From that point on, it happened so fast, I flew out of the seat and hit my head."

Wickett said she stayed at the scene and tried to help. "People are just in very bad shape," she said.

"The people that were hurt, the ones that could speak, were calling back as we called out to them," she said. "Lots of people were upset and crying, but there were no screams."

At Howard University Hospital, Johnnie Ford, an emergency-room doctor, said a 14-year-old girl suffered two broken legs in the accident. A 20-year-old male patient "looked like he had been tumbled around quite a bit, bumps and bruises from head to toe," Ford said.

Homeland Security Department spokeswoman Amy Kudwa said last evening that federal authorities had no indication of any terrorism connection.

"I don't know the reason for this accident," Metro's Catoe said. "I would still say the system is safe, but we've had an incident."