BAD AIBLING, Germany — Two commuter trains crashed head-on Tuesday morning in southern Germany, killing at least nine people and injuring around 150, slamming into each other on a curve without braking after an automatic safety system apparently failed to stop them, the transport minister said.

The first rescue units were on the scene within three minutes of receiving emergency calls, but with a river on one side and a forest on the other, it took hours to reach some of the injured in the wreckage. Rescue crews using helicopters and small boats shuttled injured passengers to the other side of the Mangfall river to waiting ambulances.

Authorities said they were taken to hospitals across southern Bavaria.

“This is the biggest accident we have had in years in this region and we have many emergency doctors, ambulances and helicopters on the scene,” police spokesman Stefan Sonntag said.

German rail operator Deutsche Bahn said safety systems on the stretch had been checked as recently as last week, but Transport Minister Alexander Dobrindt suggested a system designed to automatically brake trains if they’ve accidentally ended up on the same track didn’t seem to have functioned properly.

Dobrindt, however, said it was too early to draw conclusions.

“The site is on a curve, we have to assume that the train drivers had no visual contact and hit each other without braking,” Dobrindt told reporters in Bad Aibling, near the crash scene, adding that speeds of up to 100 kph (60 mph) were possible on the stretch.

Black boxes from both trains had been recovered and are now being analyzed, which should show what went wrong, Dobrindt said.

“We need to determine immediately whether it was a technical problem or a human mistake,” he said.

The two regional trains crashed before 7 a.m. (0600 GMT) on the single line that runs near Bad Aibling, in Bavaria, and several wagons overturned. The two train drivers are thought to be among the dead, and 50 of those hurt were being treated for serious injuries, authorities said.

Each train could hold up to 1,000 passengers and are commonly used by children traveling to school, but because of regional holidays to celebrate Carnival, fewer than 200 were on board in total.

“We’re lucky that we’re on the Carnival holidays, because usually many more people are on these trains,” regional police chief Robert Kopp said.

About 700 emergency personnel from Germany and neighboring Austria were involved in the rescue efforts and about a dozen helicopters were used.