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 agreed 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. A security official, speaking on condition of anonymity because he was not authorized to comment on the matter, said the plot was centered on the Frankfurt airport and that three of the suspects were apprehended in Hesse state, where it is located. 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. Under German law, authorities must release suspects after a maximum of 48 hours unless they have enough evidence to convince a judge that they can be held in long-term investigative custody. Prosecutors would not elaborate on the circumstances of the ongoing case, nor give any more details about the suspects. They said nine apartments were searched in Hesse and the neighboring state of Rhineland-Palatinate. 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. In a similar investigation, police in the northern city of Hamburg in 2002 apprehended seven suspected Islamic extremists who were believed to be plotting new terrorist attacks, only to release them several hours later. Authorities there later said that through five months of surveillance they had not managed to come up with enough evidence to charge the men, but that they were convinced they were getting ready to act and wanted to thwart their plot. 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.