The arrest of August Hanning, Germany's former foreign intelligence chief, has sparked calls to reform how the country's spy services are overseen.

Hanning, 80, led the BND foreign intelligence agency from 1998 to 2005. Federal prosecutors suspect him of treasonous espionage and obtaining state secrets. He was arrested at his home in Nordwalde, near Munster, last Tuesday.

Prosecutors say that in 2010, Hanning agreed to pay his former chief of staff, named only as Manfred D., to pass him BND material. Over 12 years, the aide allegedly supplied about 2,000 documents, some of them classified. Prosecutors say Hanning used the material for clients of his private consulting firm. They also say he shared political information with a representative of a foreign intelligence service for years.

Manfred D. was also arrested and charged with aiding attempted treason. Both men are in pretrial detention. Hanning has denied, through his lawyers, passing confidential documents to foreign powers. If convicted, he could face 10 years in prison, The Guardian reported. Prosecutors have not named the foreign service.

"This is an incident that has the potential to escalate into a genuine national scandal," said Konstantin von Notz, a member of parliament's intelligence oversight committee. He warned of a "devastating" loss of trust among partner countries.

Former BND vice president Arndt Freytag von Loringhoven called the files the "crown jewels of the BND." Many are believed to be briefing papers for top officials, including the chancellor.

Experts say no former spy chief of his rank has been arrested before in a western democracy.

What we know: Hanning and his former aide are in custody, accused of moving about 2,000 BND documents over 12 years.
What we don't: Which foreign service received information, and when charges will go to trial.