The head-on collision of two commuter trains in southern Germany early Tuesday left 10 people dead, dozens injured and one person missing, police said.
Eighteen people were seriously injured and another 63 sustained light wounds.
The accident, which occurred near the Bavarian town of Bad Aibling at 6:50 am (0550 GMT), saw two commuter trains carrying a total of 150 people collide at full speed on a single-track line in a partially wooded area.
Due to a curve in the track, the drivers had no visibility before the two trains crashed into one another, causing several compartments to derail, Transport Minister Alexander Dobrindt said in a press conference.
Prosecutors said they were investigating the possibility of human error and of a technical fault.
One focus for investigators was the system that causes trains to brake automatically when they are on the wrong track to prevent head-on collisions.
A spokesman for rail operator Deutsche Bahn said the system was enabled at the time of the accident and had been tested about a week ago.
"It was frightening to see how the two trains drilled into one another - the second train was completely torn apart," Dobrindt said, adding that he was in close contact with Chancellor Angela Merkel and Bavarian premier Horst Seehofer, who had both expressed their condolences to the victims' relatives.
As part of the rescue effort, as many as 15 helicopters transported people with serious injuries to nearby hospitals and were seen lifting victims to the opposite side of the river in body bags. Those with less severe injuries were being treated at the scene.
Federal and state police, paramedics, the fire department, the lifeguard service and the Red Cross had formed a rescue operation comprising more than 500 people.
The one person missing was believed to still be among the wreckage.
A spokesman for the team of paramedics working at the scene said that favourable weather conditions, the time of day and the fact that it is a holiday in Bavaria had helped to minimize losses in the accident.
Two black boxes from the trains were being examined for information about the reason for the collision. A third was still missing in the wreckage.
Deutsche Bahn said that two cranes had been ordered to remove the wreckage beginning Wednesday and that the effort would take several days.