A backpack was later blown up by German police
Passengers have been evacuated from a UK-bound plane via emergency slides after concerns over an onboard conversation that included "terrorist content".
The easyJet flight from Slovenia diverted to Germany where it made an unscheduled landing on Saturday evening.
A backpack was later blown up and a witness said two men were led away from the aircraft in handcuffs. Reports later said a third man had been detained.
German police said passengers reported the men had been having a discussion that included "terrorist content".
They are being questioned "on the suspicion of the preparation of a serious state-threatening violence", said a statement.
Some 151 passengers left flight EZY3246 on the emergency slides after "the pilot had been informed about a suspicious conversation on board".
The flight from Slovenian capital Ljubljana to London Stansted diverted to Cologne-Bonn airport at about 6.30pm (5.30pm Irish time) on Saturday.
After being evacuated, passengers were taken to a gate to be checked by police and the airport said experts had "blown up" a piece of luggage.
A passenger on the flight said they were initially told the aircraft was diverting for "operational reasons".
He said police gathered outside the plane, which was escorted to a quiet area of the airport.
"We were told they would board to deal with a security situation," said the passenger.
"When they boarded, they went to the back and took off two men plus two other passengers, who were the ones that reported the two men to cabin crew.
"The two men were questioned outside separately, then handcuffed and taken away and police wanted us off the plane quickly, so we had to use emergency slides."
He said there was no panic on board and everyone was briefly questioned by police and told they would be put up in a hotel.
However, the passenger said it was 3.00am on Sunday before they were finally provided with a room.
Take-offs and landings were suspended for three hours during the evening security alert, causing 10 flights to be diverted and delays to 20 outbound flights.
British carrier EasyJet said the captain had landed "as a precaution to enable the aircraft to go through additional security checks in Cologne where the aircraft was met by the police".