Peter Beuth, interior minister for the state of Hesse, announced Thursday that he would appoint a “special investigator” to try to shed light on the case.

Beuth said a Frankfurt police computer had been used to search for personal data on Janine Wissler of the left-wing Die Linke party, who has received several threatening letters and emails since February.

The letters were signed “NSU 2.0”, a reference to the German neo-Nazi National Socialist Underground group that committed several racist murders in the 2000s.

Seda Basay-Yildiz, the Frankfurt lawyer who represented the victims' families in the NSU trial, has also received emails with the same signature, as have other politicians, including members of the ruling conservative CDU party.

Beuth said there was “no evidence” of a network of right-wing extremists in the police.

However, the discovery that the computer had been used to search for one of the victims “feeds suspicion,” he admitted.

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“The suspicion weighs heavily,” he said. “I expect the Hesse police to leave no stone unturned in their efforts to dispel this suspicion.”

The case comes as the German police and army find themselves under increased scrutiny for extreme right-wing and racist views in their ranks, a debate fuelled by the Black Lives Matter protests in the United States.

Interior Minister Horst Seehofer has come under fire for refusing to conduct a study on racial profiling in the police, despite backing for the idea from the Justice Ministry.

However, the BfV domestic intelligence agency announced on Thursday that it would issue a report by the end of September on extremist tendencies in all the country's security forces.