Six people are under investigation in Germany over an alleged terrorist plot to blow up a commercial aircraft, prosecutors said Monday. 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. 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. 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.