"Investigations carried out up until now have not given us any clues that there was a terrorist background to the attack," prosecutors said in a statement.
Letters left at the scene had suggested Islamist militants were behind the bomb attack.
Prosecutors said they believed suspect Sergei V., a German-Russian dual national who was arrested on April 21, carried out the attack "purely for monetary reasons".
(Reporting by Michelle Martin; Editing by Andrea Shalal)
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