Authorities investigating death threats sent to a prominent German-Turkish lawmaker suspect that they were written by police officers, German media reported on Wednesday.

Axar.az reports citing AA that, Seda Basay-Yildiz, who represented families of victims killed by a neo-Nazi terror cell, received serious death threats in recent months, after she criticized the constant failure of the intelligence and police to resolve the murders.

Investigators who analyzed the fax messages sent to Basay-Yildiz concluded that they were most likely written by personnel working for the Hesse state police, as they included information and special acronyms only known by such personnel, the daily Suddeutsche Zeitung reported.

Following two serious death threats late last year, Basay-Yildiz received this month several other fax messages which included racist insults and threats.

The death threats included her address and names of her family members, which could be obtained only from official records.

In December, five police officers in Frankfurt were suspended on suspicion that they took personal information of Basay-Yildiz from police records and shared them with the far-right extremists.

The German-Turkish lawmaker represented the families of victims who were killed by the far-right National Socialist Underground (NSU).

The shadowy NSU group killed 10 people, including eight Turkish and one Greek immigrant as well as a police officer between 2000 and 2007, but the murders have remained unresolved.

The German public first learned of the group's existence in 2011, when two members -- Uwe Mundlos and Uwe Bohnhardt -- died after an unsuccessful bank robbery.

The group’s only surviving member Beate Zschaepe was given a life sentence last year, but families of the victims expressed disappointment with the verdict and said the five-year-long trial has left many key questions unanswered.