The suspects had repeatedly made contact with the person since first approaching him during the summer but were unable to agree on a price for planting a bomb, the prosecutor said.

Die Welt newspaper said the person was a male employee of Frankfurt airport.

None of the names or details of the suspects have been made public, but a security source told Reuters news agency that they were from Jordan, Kuwait and Iraq.

Der Tagesspiegel newspaper, however, reported that most were Jordanians of Palestinian origin.

"The suspects were temporarily taken into custody and informed of the suspicions against them before they were released on Saturday, with the exception of one wanted in connection with another crime," the prosecutor said.

Suspects released

The men are suspected of being members of a "terrorist organisation", the office said. It did not explain why the suspects, who remain under investigation, had been released.

"I can't comment on the particularities about the decision to release them," said a spokesman for the interior ministry. Germany's BND foreign intelligence agency also declined to comment.
   
The prosecutor's office said it had searched nine apartments on Friday in the western German states of Rhineland-Palatinate and Hesse to gather evidence.

The prosecutor's office gave no details of which plane or airport was to have been involved.

Monika Harms, Germany's federal prosecutor, last week said that although no attack had taken place on German soil recently, the apparent peace "could be deceptive" and the country must remain vigilant.

On July 31, home-made bombs hidden in suitcases were discovered on two passenger trains heading for the western cities of Hamm, near Dortmund, and Koblenz. Two Lebanese men have been charged with planting the devices.