"This data breach of hundreds of German politicians is alarming, but at the same time it's not surprising", said Mike Hart at commercial cybersecurity firm FireEye told the Reuters news agency, citing previous hacks.
The leak affects politicians part of all of Germany's left and centrist political parties, such as the CDU, CSU, SPD, Greens, Left Party, and the FDP.
"Hacking attack against politicians: The BSI is now intensively probing the issue in close cooperation with other federal institutions", the BSI said on Twitter, adding that "according to what we know so far" the government's confidential networks were unaffected.
"We have been dealing with the issue since yesterday evening and are now informing our people", an SPD spokesman told German news agency dpa, adding that the party had been in contact with the relevant authorities.
"We do not know who is behind this attack or where the stolen data comes from".
It was not immediately clear whether the officials were targeted by hackers or were the victims of an internal leak.
ARD reported earlier that the data, which was published on a now-blocked Twitter account, included addresses, personal letter and copies of identity cards.
Other reports listed photos of ID cards, direct debit documents and details of family members' credit cards as being among the documents.
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The leak has affected also celebrities and journalists.
Links to the data appeared daily on the Twitter account in December in an advent calendar-style, but it was only noticed more recently.
The leaks began with data from German TV personalities Jan Böhmermann and Til Schweiger, Youtube star LeFloid and rapper Sidos. The majority of them only featured private data and didn't include anything relating to politics.
Discovered only yesterday, the information had actually been released over the past fortnight by Twitter user "G0d", who claims to be based in Hamburg and whose biog indicates is a security researcher with an interest in "satire & ironie".
A motive for the leak remains unclear, as does how it could have stayed unnoticed for more than 10 days over the Christmas break.
The data was spread via two Twitter accounts named @_0rbit and @_0rbiter, according to security researcher Luca Hammer.
The Federal Office for Information and Technical Security (BSI) had confirmed the data breach, saying it liaises with other agencies on the issue. In 2016 the defence ministry set up a cyber department to coordinate a response to online intrusions.