An employee of Germany's foreign intelligence service has been arrested on suspicion of treason for allegedly passing state secrets to Russia, German prosecutors said Thursday.

The man, a German citizen who was identified only as Carsten L. in line with German privacy rules, was arrested in Berlin on Wednesday, federal prosecutors said in a statement.

The apartment and workplace of the employee of the Federal Intelligence Service, or BND, and of another person were searched, prosecutors said.

"The accused is suspected of state treason," the federal prosecutors' office said. They said the suspect passed this year "information that he had acquired in his professional activity" to a Russian intelligence service, which they didn't identify.

They added that the information was a "state secret" as defined by Germany's criminal code, but did not elaborate.

The BND had started its own internal investigation as soon as it became aware of the possibility of treason within its own ranks, BND head Bruno Kahl said. When these suspicions hardened up, it called in federal prosecutors.

Discretion was key going forward, as any detail of the investigation that was made public could give the "opponent an advantage in its intent to harm Germany," he said.

"With Russia, we are dealing with an actor where we must reckon with its ruthlessness and willingness to be violent."

The suspect was brought Thursday before a judge, who ordered him held in custody pending a possible indictment, prosecutors said. The investigation was being conducted "in close cooperation" with the BND, they said.