German prosecutors are investigating a suspected neo-Nazi cell within the Frankfurt police force that is thought to have sent a German-Turkish lawyer a message threatening to kill her two-year-old daughter.

Five police officers – four men and a woman – have been suspended on suspicion of exchanging far-right and racist messages in a chat group and sending the note to the lawyer, Seda Basay-Yildiz, this summer.

The anonymous message, which arrived by fax at Basay-Yildiz’s law firm in August, told her to leave Germany or risk the “slaughter” of her daughter. The note included the lawyer’s home address and was signed “NSU 2.0”, apparently a reference to the National Socialist Underground terror cell responsible for the murder of 10 people – nine of them from immigrant backgrounds – in Germany between 2000 and 2007.

Basay-Yildiz represented one of the victim’s families in the five-year trial of the sole surviving NSU member, Beate Zschäpe, who in July this year received a life sentence.

Basay-Yildiz has previously defended suspected Islamists, and she told the Frankfurter Allgemeine Zeitung (FAZ) paper she was used to receiving threats. However, the fax was the first to include her home address and to threaten her child.

A police internal investigation found that an officer’s computer at a station in Frankfurt had accessed a confidential database to obtain Basay-Yildiz’s address shortly before the fax was sent. Phones belonging to police officers on duty at the time were confiscated, revealing that a number of them had been exchanging far-right and racist messages in a chat group.

The officers have now been suspended pending the results of an investigation by Hesse’s state criminal office.

Basay-Yildiz has complained that the police did not keep her informed after she filed a complaint over the fax. She read about the internal investigation in the German press at the weekend.

“I would have liked the police themselves to have informed me ahead of time,” she told the FAZ.

The case will be seen as further evidence of pervasive far-right sympathies rooted deep in parts of the German state. In September the domestic intelligence chief Hans-Georg Maaßen resigned after he claimed videos showing far-right violence in Chemnitz were fake.

In a separate case, the leadership of the far-right Alternative for Germany (AfD) party was meeting in Berlin on Monday to decide whether to expel a high-ranking member suspected of belonging to a neo-Nazi organisation. The AfD claims it wants to clear its ranks of far-right extremists in an attempt to avoid the entire party being put under surveillance by German intelligence.

Doris Fürstin von Sayn-Wittgenstein, a former chair of the AfD’s Schleswig-Holstein chapter, was removed from the party’s regional parliamentary group after admitting she had endorsed Verein Gedächtnisstätte, a group founded in 1992 by a convicted Holocaust denier, Ursula Haverbeck-Wetzel.

The group says it wants to honour German victims of the second world war, but in 2017 German intelligence placed it under surveillance amid claims it was working to subvert democracy. The AfD has since included Verein Gedächtnisstätte on a list of groups its members are forbidden from joining.