Hanning, who led the BND until 2005, was taken into custody at his home in Steinfurt county, near the Dutch border, on Tuesday morning under orders from Germany's federal prosecutor. During a simultaneous operation in Bavaria, a senior BND employee identified only as Manfred D under German privacy laws was arrested on suspicion of aiding treason.
Prosecutors allege the two men agreed in spring 2010 for Manfred D to continuously transmit valuable intelligence from the BND to Hanning in exchange for payment. They also claim Hanning drafted an analysis for an officer of a foreign intelligence service, though it remains unclear whether the material ever reached its recipient.
The arrests mark the first time a former BND president has been charged with espionage. The investigation covers 16 years of alleged illegal document collection and the sale of information to an unidentified foreign power. German authorities have not disclosed which country is believed to have received the secrets, nor the full extent of the compromise.
Hanning was known as "Mr Security" during his tenure at the BND, where he served from 1998 to 2005. The case has sent shockwaves through Germany's intelligence community, raising urgent questions about how a former top spy could operate undetected for so long.