At least nine people have been killed and 70 injured after two subway trains collided in Washington DC during the afternoon rush hour.
Rescue workers located three more bodies in the wreckage late last night.
Mayor Adrian Fenty called the crash the deadliest in the 33-year history of the city's Metro subway system.
Fire Chief Dennis Rubin said 70 people had been treated for injuries, including at least two who were critically injured.
At least one car from one of the trains was hurled onto the top of the other train in the accident, which occurred on above-ground tracks.
Emergency workers propped ladders up against train doors to rescue people.
Officials said one train hit another train that had stopped, but the cause of the crash was not immediately clear.
President Barack Obama and his wife, Michelle, sent condolences.
"Michelle and I were saddened by the terrible accident in Northeast Washington, DC, today," Mr Obama said in a statement. "Our thoughts and prayers go out to the families and friends affected by this tragedy."
Federal investigators will lead an inquiry into the incident, which occurred on the heavily travelled red line at 5pm 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 at approximately 5:02 or 5:03 (was) one train was stopped waiting to get the order to pass because of a train stopped at the platform. 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, told reporters.
The transit authority said one of those killed was a female train operator in the trailing train. The accident trapped passengers in one or more of the subway cars.
"It just happened. There was no slowing down of the train, just a jerk," said passenger Theroza Doshi. "There was no attempt at braking. We just slammed into whatever we slammed into."
Emergency rescue personnel worked to free the passengers, while others carried injured people off the trains on stretchers.
Authorities said the accident would affect freight train traffic operating through Maryland and Washington.
One witness described how one train appeared to collide with - and then run up and over - the second train.
"It was very mangled, everything is ripped out of there," the woman, who was not identified, told the local ABC television affiliate.
A reporter with Fox television's local news affiliate said he saw what appeared to be a body covered with a sheet, and several injured passengers, including one wearing a neck brace.
The subway system is heavily used to get in and around the US capital.
There have been relatively few serious accidents involving commuter trains in the United States over the past 15 years. In one accident in September, a commuter train collided with a freight train in Los Angeles, killing 25 people and injuring more than 130. The engineer of the commuter train was found to have been sending text messages on his mobile phone seconds before the crash.
Investigators with the National Transportation Safety Board were at the crash site yesterday to collect evidence.
"They will have to do both an investigation and then a release of the scene for us to clean it up," Mr Fenty told reporters at the scene.
Agencies