Three men were held by police after the pilot of the Airbus A319 took the decision to make an unscheduled landing at Cologne-Bonn airport, in Germany, at around 5pm on Saturday.
All 151 passengers on the flight from Ljubljana, in Slovenia, were evacuated down emergency chutes and several people suffered minor injuries, Cologne police said.
The three suspects, aged 31, 38 and 48, were questioned along with the crew and 17 passengers at a police station in the city.
In a joint statement, police and the prosecutor's office said the plane had been released after an intensive search and the men were not known to authorities.
The older man had been carrying a backpack and officers searched the bag, believing it was "suspicious", and found cables. Witnesses also claimed to have seen a book featuring a precision rifle, police said.
An earlier statement said: "During the flight, other passengers had made the crew aware of the men.
"According to the witnesses, these had been the subject of terrorist content."
The three men continue to face questioning and police said they were investigating "on the suspicion of the preparation of a serious state-threatening violence".
A spokesman for Cologne-Bonn airport said the pilot took the decision to land the plane.
"Prior to this, 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 of the aircraft of the type A319, the 151 passengers left the machine via emergency slides and were taken to a transit gate.
"The police have been informed and have taken action.
"Thus, passengers had to undergo a check immediately after landing."
Flights were suspended at the airport for three hours.
Passengers tweeted that they had returned to the UK on Sunday afternoon.
One said: "Just landed and so glad to be home. What a crazy 12 hours."
An easyJet spokeswoman said: "easyJet can confirm that flight EZY3246 from Ljubljana to London Stansted diverted to Cologne on Saturday.
"The captain took the decision as a precaution to enable the aircraft to go through additional security checks in Cologne where the aircraft was met by the police.
"In compliance with the local authorities' guidance, passengers disembarked to allow additional security checks to be performed."