A flight from Ljubljana, Slovenia, to London had to make an unexpected stop in Germany and passengers evacuated after a suspicious conversation was overheard on board.
Witnesses reported hearing several men having a conversation with "terroristic content", according to Cologne police spokesperson Christoph Gilles.
There were also concerns about a "suspicious object in the shape of a backpack".
It prompted a diversion to the Cologne-Bonn airport in Germany and the 151 passengers were evacuated through the emergency slides in a secured area.
Three men have since been detained and a backpack linked to them has been destroyed by explosives experts, according to Mr Gilles.
More than a dozen flights were delayed due to the incident and another 10 had to be diverted to other airports.
An investigation is ongoing.