A Lufthansa flight to Chicago was forced to return to Frankfurt on Thursday after an unusual odour was smelled in the cabin, the German airline said on Thursday.
The Boeing 747 carrying 320 passengers and 19 crew members turned around due to an "unidentifiable electrical odour," a company statement said.
Lufthansa said there was no smoke on board at any time and that the cause of the smell was still being investigated by technicians.
The aircraft landed "normally" in Frankfurt.
The passengers were to continue their journey on Thursday evening on a replacement aircraft.
Germany's Bild newspaper reported that the smell was detected about 90 minutes after departure and that the plane was already over the Atlantic when pilots decided to return to Frankfurt.