A plane flying from Slovenia to the UK made an unplanned landing after the pilot was alerted to a "suspicious conversation" with "terrorist content".
Three British men arrested after the Easyjet flight from Ljubljana to Stansted, in Essex, was diverted to Germany's Cologne-Bonn airport on Saturday are being questioned.
All 151 passengers were evacuated and flights were suspended for three hours.
A backpack belonging to the men was blown up by police.
A spokesman for Cologne-Bonn airport said: "The pilot had been informed about a suspicious conversation on board, after which he decided to make an unscheduled landing in Cologne-Bonn.
"After the safe landing... the 151 passengers left the [aircraft] via emergency slides and were taken to a transit gate."
Nine people received medical treatment after using the slides on the Airbus 319.
A statement from Cologne police said other passengers had made the crew aware of a conversation by three men.
It said the subject had been "terrorist content", without elaborating further.
Easyjet said the captain had taken the decision to land as a precaution to allow additional security checks to take place.
The diverted plane landed in London Stansted airport on Sunday.
Passenger Daniel Noonan told the BBC that armed police entered the aircraft and escorted two men off. The third was led away on the tarmac after the passengers had exited via the slides.
Passenger Richard Peters said he saw the two Asian men being handcuffed on the tarmac at Cologne-Bonn airport.
He added that people on the flight were quarantined for about nine hours while police interviewed passengers about the men.
He said: "To be fair spirits were quite high considering what had happened.
"In the current climate you have to take everything seriously. My suspicion is it was, hopefully, nothing sinister. But you have to take everything seriously.
"The right thing was done."
Another passenger, Dave Hargreaves, said: "In the flight I didn't see the suspects at all. But I noticed quite a bit of activity, with aircrew popping into the cockpit repeatedly. And a slower than normal serving of the food and drink service.
"Then at our cruising altitude I was surprised to see the air brakes go up on the wings and us start a rather speedy descent. It wasn't until perhaps 10 minutes later that we were told 'due to operational reasons' we would be diverting to Cologne airport."
A backpack belonging to the men was "blown up in a controlled manner", police said.
No explosives were found in the passengers' luggage or on the plane.
The men are aged 31, 38 and 48, a Cologne police spokesman said. It is unclear if charges will be brought.
The Bild newspaper said the men worked for a London firm and were returning from a business trip.
All the other passengers were given a hotel for the night, an Easyjet spokeswoman said, adding: "We thank passengers for their understanding. The safety of Easyjet's passengers and crew is our highest priority."