Members of the families of two men who were killed by the right-wing terror cell the "National-Socialist Underground" (NSU) are now suing over multiple investigative failures, German media reported on Sunday.

The district court in Nuremberg confirmed to news agency DPA that two cases filed against the German federal government and the states of Thuringia and Bavaria are pending.

German public broadcaster ARD also reported that the plaintiffs are family members of the NSU's first murder victim, flower-seller Enver Simsek, and the group's sixth victim, kebab shop owner Ismail Yasar.

Simsek and Yasar were allegedly shot dead by two of the NSU's members, Uwe Mundlos and Uwe Böhnhardt, in Nuremberg in 2000 and in 2005, respectively.

The NSU carried out a series of xenophobic and violent crimes between 2000 and 2007, including 10 murders, two bombings and 15 bank robberies.

A third NSU member, Beate Zschäpe, is accused of being involved in the crimes and for helping cover the men's tracks after the murders. Mundlos and Böhnhardt were found dead in November 2011 in an apparent murder-suicide.

"First, the Nazis destroyed our lives because the state could not or did not want to protect us," Simsek's son Abdulkerim told DPA.

"Then the state betrayed us once more," he said in reference to how the police treated his family following his father's murder. Investigators have been accused of being slow to follow leads pointing toward far-right involvement in the murders and criticized for limiting investigations to organized crime ties among immigrants to Germany.

Throughout Zschäpe's Munich trial - now in its fourth year - and several parliamentary inquiries, it became apparent that authorities wasted years chasing false leads and putting the victims' relatives under surveillance.

The families of Enver Simsek (top left) and Ismail Yasar (second from left on bottom) are seeking justice from the German state

Lawyer: Deaths could have been prevented

Both families are seeking 50,000 euros ($55,900) per family member in damages. The lawyer representing both families, Mehmet Daimagüler, told DPA that the complaints are based on "mishaps in the search for the underground NSU trio."

Daimagüler argued that both federal and state authorities could have arrested Mundlos, Böhnhardt and Zschäpe in 1998 and at the latest in 2000, thereby preventing the subsequent murders.

Furthermore, the state gave hundreds of thousands of euros to informants who used the money to support the neo-Nazi trio.

"Our case is based on the findings of numerous investigations" both from parliamentary inquiries and the evidence presented in the ongoing trial against Zschäpe in Munich, Daimagüler said.

The families of other NSU murder victims are expected to file similar complaints. Daimagüler added that should the case prove too difficult in German court, he is prepared to take the case to a foreign court and sue the German government from there.