FILE PHOTO: WikiLeaks founder Julian Assange makes a speech from the balcony of the Ecuadorian Embassy, in central London, Britain February 5, 2016. REUTERS/Peter Nicholls/File Photo

BERLIN (Reuters) - Berlin’s chief public prosecutor has extended an investigation into the release of a trove of documents by WikiLeaks to include the chancellery as well as the Bundestag lower house of parliament, broadcaster NDR said on Thursday.

Last December, WikiLeaks released the confidential documents, which German security agencies had submitted to a parliamentary committee investigating the extent to which German spies helped the U.S. National Security Agency (NSA) to spy in Europe.

The extension of the investigation to include the chancellery did not necessarily mean the Berlin public prosecutor had firm suspicions that individuals at Chancellor Angela Merkel’s office were involved in the leak, NDR said.

Government sources told Reuters that the chancellery had agreed several weeks ago to the investigation “against unknown” persons, to allow the inquiry to proceed. There were no firm suspicions against chancellery officials, the sources added.

Surveillance is a sensitive issue in Germany where East Germany’s Stasi secret police and the Nazi era Gestapo kept a close watch on the population.

Merkel told the parliamentary committee in February that she did not know how closely Germany’s spies cooperated with their U.S. counterparts until 2015, well after an uproar over reports of U.S. bugging of her cellphone.