Two helicopters clipped each other and crashed as they landed in a snowstorm near Berlin's Olympic Stadium during a federal police exercise, leaving one person dead and several injured.
One of the helicopter pilots died at the scene, officials said. Five people were injured, four of them seriously.
It wasn't immediately clear whether the injured people were in the helicopters.
Pictures from the scene showed one of the helicopters lying on its side in the snow in a field behind the stadium and the other next to it, still upright.
Eyewitness Johannes Malinowski said on n-tv television that he saw three helicopters approaching and that the snow on the field was being kicked up by the aircraft, so "you couldn't see a whole lot anymore".
Then "there was a big bang and someone shouted, 'everybody on the ground,'" he said. "And then we looked up and there was blood on the ground."
The police were conducting a training exercise on dealing with football violence.
The Olympic Stadium is home to the Hertha Berlin football club. It also hosts the annual German Cup final and was the venue for the 2006 World Cup final.