Prosecutors in Verden, Lower Saxony, are looking into how thieves got hold of access information for 18 million email accounts, a spokesman for the state prosecutor's office told The Local.
All major German internet providers are affected by the theft and some accounts were accessed, Spiegel Online reported.
But it is unclear how many of the 18 million accounts were German as some had international domains such as .com.
However, the information stolen was current rather than historic.
It is the second major data theft to emerge in the last few months.
In January the Federal Office for Online Security (BSI) revealed hackers had accessed 16 million email accounts and, according to Spiegel, the two cases are likely to be linked.
The prosecutor’s office in Verden would not comment further to The Local on how many of the accounts were actually accessed or how many of the email addresses were from outside of Germany.
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