BERLIN — At least six people are under investigation in an alleged terrorist plot to blow up a commercial aircraft, German prosecutors said Monday.

The six 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 load a case or bag containing explosives onto a plane in exchange for an unspecified payment, it added.

Prosecutors said the accused then contacted the plot's alleged backers but were unable to agree on the value of the promised reward.

The six were detained on Friday, and five were released Saturday after questioning. The remaining suspect was held in an unrelated matter.

Prosecutors said others are suspected of involvement. They gave no information on the six suspects.

Prosecutors said nine apartments in the southwestern states of Rhineland-Palatinate and Hesse were searched.