The national train operator was being probed on suspicion that it had negligently caused bodily harm and that train staff had failed to adequately assist passengers, a spokeswoman for the federal police in St. Augustin said late on Sunday.

Temperatures reached up to 50 degrees Celsius on the train from Berlin to Cologne – one of three trains over the hottest weekend of the year to have suffered air-conditioning malfunctions.

Nine students and elderly passengers were taken to the hospital, where some received IV drips. The student group from Remscheid and Willich in North Rhine-Westphalia had been on its way back from a class trip to Berlin.

Witnesses described dramatic scenes on board the ICE train as dehydrated students reportedly lay down in the aisles, and one woman smashed a window during the trip in hopes of getting fresh air.

The police spokeswoman said it had become known before Bielefeld, the Rhineland city where the train finally stopped, that the air-conditioning system was broken. A witness told a train conductor that he had noticed a sharp smell of burning rubber. The conductor subsequently established that the air-conditioning was no longer working, yet the train continued on its journey.

Whether the break-down had already been noticed in Berlin, where the train originated, had yet to be established, the spokeswoman said.

Deutsche Bahn boss Rüdiger Grube phoned students and teachers on Sunday to apologise. The operator has also offered compensation to the people affected by the baking conditions.

But passengers’ rights group Fahrgastverband Pro Bahn chairman Hartmut Buyken, told broadcaster hr info that the breakdown was evidence of systemic failures.

“Deutsche Bahn has been penny-pinching in the wrong places,” he said.