The investigators found a list of the real names, identities and aliases of German foreign intelligence agency officers stored on the hard drive of a US double agent who was uncovered and arrested this past summer.
The name of the agent has not been released and was identified only as Markus R due to Germany's strict privacy laws.
The list is believed to contain the real identities and aliases of up to 3,500 German officers working for the Bundesnachrichtendienst (BND), Germany's foreign intelligence agency, abroad. Investigators fear that the information might have been passed to foreign intelligence agencies.
Markus R was arrested last summer after being uncovered as a double agent working both for the BND and CIA.
He has reportedly confessed to passing the CIA more than 200 secret documents over a period of two years, in return for payments of €25,000 ($29,460).
Markus R worked in the registry section of BND’s overseas operations department, which is responsible for the protection of soldiers serving abroad. There, he had access to top secret documents, including the identities of operatives posted abroad.
The-then 31-year-old BND employee was described as "walk-in agent" — someone who presents himself on his own to a foreign spy service and dangles an offer of secrets, according to Reuters.
He approached the CIA by email in 2012, offering to provide them with information. However, he was caught only in 2014 after he sent a similar email to the Russian Embassy in Berlin offering to sell secrets in exchange for cash, which was intercepted. At the time, the German investigators were stunned at his confession that he had been spying for the US.