The pistols were found hidden in the forest last year after someone came forward with a tip, public service radio and Süddeutsche Zeitung jointly report.
German police authorities suspect that they were hidden to be used for acts of violence on behalf of Russia. An investigation is underway into suspected preparations for a "serious act of violence" with consequences for the country's security.
A person is being held in custody in Romania on suspicion of hiding the weapons, Germany's interior minister confirmed after the media revelations. The person is suspected of being a low-level agent and is still being officially investigated for links to foreign powers.
