Six people are under investigation in Germany over an alleged terrorist plot to blow up an aircraft, German prosecutors said as Israeli government sources confirmed that an El Al plane had been the target. "We received information on the investigation and turned to the German authorities to receive continuous updates," Transportation Minister Shaul Mofaz's spokesperson Timor Dill told The Jerusalem Post. "Our security forces are investigating." Sources in the Shin Bet (Israel Security Agency) also said they were being updated by the German authorities. 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, German 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 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 said the plot was centered on the Frankfurt airport and the intention was to attack a plane belonging to El Al. Three of the suspects were apprehended in Hesse state, where the airport is located, the official said. Dani Shenar, head of the security department at the Transportation Ministry, said a security alert was in effect for Frankfurt but did not extend to El Al operations around the world. He said security at Frankfurt airport was very good, adding that his office was awaiting clarification about the investigation before deciding if it was necessary to send additional personnel to beef up operations there. Shenar said he was unaware of reports that the arrests were the result of an Israeli tip-off to German security authorities over a month ago. El Al spokeswoman Amalia Glaser declined to comment. The six suspects, 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 had not been 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. A spokesman for Germany's Interior Ministry, Stefan Kaller, declined to comment on details of the alleged plot, its timing, what specific airport was involved or why the suspects were released, citing 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 September 11, 2001, attacks on the United States, when it emerged that three of the suicide pilots 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. Yaakov Katz contributed to this report.