Merkel's office is being investigated over documents published by Julian Assange's Wikileaks website
Berlin's chief public prosecutor was extending its inquiry to include the chancellor's own office and the Bundestag lower house of parliament, according to broadcaster NDR.
The investigation focuses on Wikileaks' release of secret reports submitted by German security agencies to a parliamentary committee investigating the extent to which German spies helped the US National Security Agency (NSA) to spy in Europe.
Although the remit of the prosecutor's investigation had been extended, it did not mean there were firm suspicions that individuals inside the chancellery were involved in the leak, which took place in December, the broadcaster 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.
The parliamentary inquiry in question was launched following the Edward Snowdon leaks of 2013, which revealed countless surveillance programmes carried out globally by the NSA.
The NSA is said to have been helped spy on European allies by German security agencies
Among the revelations were reports US spooks tapped Chancellor Merkel's own mobile phone.
Further leaks revealed the extent to which the German BND intelligence agency had apparently helped the NSA spy on European allies.
Many of the committee papers leaked last year were classified "only for official use" – the lowest level of secrecy, NDR reported.
They were available to all MPs on the parliamentary committee and their deputies, staff and spokespeople.
Prosecutors have no "firm" suspicions that an individual from Merkel's office was responsible
Many of the workers had made copies of the inquiry's entire database, it was reported.
Until now, the leak was thought to have come from someone in parliament.
But now the ring of suspicion has grown to include government workers.
Ms Merkel told the parliamentary committee in February that she did not know how closely Germany's spies cooperated with their US counterparts until 2015, years after an the uproar over her mobile phone bugging controversy.