The six, as well as other people who have not yet been identified, are believed to have begun preparations for an attack on behalf of "so far unknown" terrorist backers, federal prosecutors said in a statement.
The plot to smuggle a bomb onto a plane seems to have taken place in the summer, when Germany was hosting the soccer World Cup, which brought millions of people into Germany, reports CBS News' Peter Bild. Since then, there have also been two bombs on trains, which were detected before they went off.
Several of the accused approached a person with security clearance at an unidentified airport last summer, the statement said. That person said he or she was prepared to smuggle a case or bag containing explosives onto a plane in an exchange for an unspecified payment, it added.
Prosecutors said some of the accused then contacted the plot's alleged backers, but were unable to agree on the value of the promised reward.
The six, who could face charges of belonging to or supporting a terrorist organization, were temporarily detained on Friday, but five of them were released Saturday after questioning. The remaining suspect was kept in custody over an unrelated matter.
Prosecutors gave no information on the six suspects.
The prosecutors clearly don't feel that they have arrested the men who were actually behind the plot, because thee suspects were released very quickly, reports Bild. These were the intermediaries, people dealing on behalf of the terrorist organization.
They said nine apartments were searched in the southwestern states of Rhineland-Palatinate and Hesse. The latter state includes Frankfurt and its airport.
Germany's biggest airline, Lufthansa, said it was not the target.
"We are definitely not connected with this investigation," spokesman Thomas Jachnow said.
Police agencies from both states, as well as German federal police, Frankfurt airport police and Frankfurt's city police all refused comment, referring questions to the federal prosecutors.
Interior Ministry spokesman Stefan Kaller declined to comment on details of the alleged plot, its timing, what specific airport was involved or why the suspects were released, pointing to the ongoing investigation.
"This case is encouraging in that our security authorities are clearly very observant, get very close to possible (terrorist) structures and, at least so far, have succeeded in intervening early enough," Kaller told reporters.
Germany stiffened counterterrorism laws after the Sept. 11, 2001 attacks on the United States, when it emerged that three of the suicide pilots in that plot had lived and studied undetected in Hamburg.
The country was shaken this summer by a failed attempt to blow up two trains — a case that brought home to many the fact that Germany itself is now also a terrorist target.
Two Lebanese men have been arrested for allegedly planting bombs on the trains at Cologne station on July 31. The bombs were found later in the day, and authorities said the detonators went off but failed to ignite the crude devices.