A spokesman for the airline told reporters that the flight from Frankfurt on Saturday with 160 people on board was carrying luggage from a passenger who was not on the plane.
As a precaution, the pilots landed the Airbus A330 in Iceland, where the baggage was removed. The flight then continued on to Detroit with a delay of about two hours.
The spokesman declined to comment on media reports that security experts feared there were explosives on board, but noted that all luggage in Frankfurt is X-rayed before being loaded onto planes.
"At no time was there a threat to security," he said.
It was not immediately clear how the luggage turned up on the plane without its owner.
The incident came as a 23-year-old Nigerian was charged in the United States with trying to blow up a Northwest Airlines flight from Amsterdam as it was preparing to land in Detroit on Christmas Day.
Airports worldwide, including Germany's largest in Frankfurt, have since tightened their security measures considerably.