Three British passengers of an easyJet flight from Slovenia to the UK were briefly detained and later released without being charged on Sunday (11 June) after the flight had to be diverted to Cologne, Germany over security concerns.

Other passengers on board the flight on Saturday complained that the three men were discussing "terrorist matters" and also carried a book entitled "Kill" with a sniper rifle on its cover, German police and state prosecutors said in a statement, according to Reuters.

"The criminal investigation against them has been halted. No evidence was found," a local police spokesman reportedly said after the men were questioned about the suspected conversation. Police also said that the alleged conversation could not be verified. "We now believe that there was never any real danger."

Bild newspaper reported that passengers had also told the flight crew that they heard the three men using words like "bomb" and "explosive" and one of them also carried a suspicious backpack. The backpack was examined by German authorities and blown up in a controlled explosion as it contained suspicious cables, the police spokesman said.

However, police said the men, aged 31, 38 and 48, worked for a British company and were returning to London after an official trip to Ljubljana.

All the 151 passengers on the flight, except the three Britons, were sent back to London on a separate flight on Sunday. However, it was not clear when the three men would be sent back, Reuters wrote.

Police also said that 17 other passengers on the flight and the easyJet crew were questioned at Cologne police headquarters. Authorities in Germany said that the recent series of terrorist incidents in the UK, including the recent London terror attacks, escalated their fears and compelled them to take extra measures to ensure passenger safety.

Following the security scare, the flight was diverted to Cologne where it made an emergency landing and all the passengers were evacuated from the Airbus 319 aircraft using emergency slides. Nine people were reported to have received medical treatment following the evacuation.

Some 17 inbound flights were diverted, 20 departing flights were delayed and two were cancelled at the Cologne airport while the investigation was ongoing, a spokeswoman for Cologne airport said. She later confirmed that air traffic had returned to normal following a three-hour interruption.