German authorities questioned three British men who were detained after their conversations on board an easyJet flight from Slovenia to London prompted the pilot to divert to Cologne, Germany, a local police spokesman said on Sunday.
Nine people received medical treatment after all 151 passengers were evacuated from the Airbus 319 aircraft using emergency slides.
No explosives were found and a spokeswoman for the airport said the remaining passengers had departed on another easyJet plane for London, the United Kingdom.
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The pilot diverted the flight from Ljubljana to Cologne on Saturday evening after passengers reported the men were discussing "terrorist matters" and carried a book with a sniper rifle on the cover, police and state prosecutors said in a joint statement.
The pilot decided to land the aircraft in Cologne after passengers told airline personnel they had heard the men using words including "bomb" and "explosive," Bild newspaper quoted a spokesman for the German federal police as saying.
The men were aged 31, 38 and 48, a spokesman for Cologne police said. Police said in a statement that they had no prior security information about the men.
The spokesman said the men worked for a British company and were returning to London after a business trip, but declined to name the company.
A backpack that belonged to the men was examined and destroyed in a controlled explosion by authorities after officials found potentially suspicious cables, the spokesman said. But no dangerous contents or explosives were found in the bag or on board the aircraft.
Police and prosecutors said they continued to investigate the men on suspicion that they were preparing to carry out "a serious violent crime that could have jeopardized the state." Mobile phones taken from the men were still being examined, the police spokesman said.
He said it was unclear if any charges would be brought against the men.
The incident meant 17 inbound flights were diverted, 20 departing flights were delayed and two flights were cancelled, a spokeswoman for Cologne airport said, adding that air traffic had returned to normal after a three-hour interruption on Saturday night.
Seventeen additional passengers and the easyJet crew were questioned about the incident at Cologne police headquarters, police said in a statement.