Hundreds of German politicians, journalists and celebrities have had personal details hacked and posted online, according to local media reports.
Parties across the political spectrum are said to have been affected, with the exception of the far-right Alternative for Germany (AfD) party.
Chancellor Angela Merkel is among those who've had data published, with both Bundestag and state officials impacted.
Broadcaster RBB reported that musicians, activists and Youtubers have also had some of their details published online.
According to Deutsche Welle, details published on social media included mobile phone numbers and contact details.
However, some other personal data - including credit card information and photo ID cards - is also believed to have been published in individual cases.
RBB correspondent Michael Götschenberg indicated that the data did not appear to include any "politically explosive" information.
Documents are believed to have been published on Twitter last month in the form of an 'advent calendar', but full details of the hacking have only emerged following local media reports on Thursday.
Justice Minister Katarina Barley described it as a "serious attack" on the right to privacy by people who want to damage trust in Germany's democracy and institutions.
She added that the perpetrators must be quickly found and their motives determined.
In a statement, the German Federal Office for Information Security said federal authorities are 'intensively' investigating the apparent hacking, with efforts being led by the National Cyber Defence Centre.
It adds that there's not believed to be any concerns about other government networks.
