Last week marked one year since the start of the trial in Munich of the last surviving member of the neo-Nazi National Socialist Underground (NSU). The anniversary of the start of the trial was overshadowed by the mysterious death of a longstanding German undercover agent, Thomas Richter, nicknamed "Corelli".

“Corelli” was due to appear in the Munich court to give evidence on the workings of the NSU. Between 1997and 2007, Richter-Correlli was an undercover agent for the the German secret service (Office for Protection of the Constitution, BfV). Some sources claim he worked for the secret service even longer—for a total of 18 years, from 1994 until his unmasking in 2012. According to the newspaper Bild am Sonntag, during this period he received 180,000 euros for his services. He must of been aware of the activities of the NSU at an early stage: he mentioned the terror group in a contribution he wrote for a far-right magazine in 2002.

The circumstances of his death are sketchy.

According to Spiegel Online, a "representative of a security authority" found the body of the 39-year-old on April 3 at his home near Paderborn. The Attorney General's office and the Federal Criminal Police (BKA) deny any involvement in the discovery of the corpse, while the BfV and the Interior Ministry have declined to give any information.

Corelli had been living under a BfV witness protection program since 2012, after he was unmasked as a longtime undercover agent within the far-right milieu in the summer of the same year. Only a handful of officials could have been aware of his location and his new identity.

The official cause of his death was a sugar shock triggered by an allegedly unknown diabetes condition. According to numerous media reports, citing "security circles" an autopsy failed to find any evidence of involvement in his death by other persons.

It is possible that Corelli had once again come to the attention of the secret services in recent months. At the end of February, Der Spiege l reported that an undercover agent had reported to the offices of the state secret service (LfV) in Hamburg and had handed over a CD labeled "NSU/NSDAP" that he had received eight years earlier from Corelli.

The Hamburg LfV reported the handover to the Attorney General, who then ordered a search of Corelli's apartment on April 25—a full three weeks after the discovery of his corpse.

According to Der Spiegel a total of around 15,000 image and text files are contained on the CD. Apparently Corelli was the author of some of the files while others were explicitly to the "National Socialist Underground of the NSDAP (NSU)".

It is reported that the CD was completed in June 2006. The responsibility of the NSU for a series of 10 murders of immigrant workers and one police official only came to light in November 2011 following the violent deaths of two NSU members, Uwe Mundlos and Uwe Böhnhardt in an alleged double suicide. The security authorities have also repeatedly claimed they knew nothing of the existence of the group until the two deaths.

Corelli, also known as "HJ-Thommy" played a central role in the far-right milieu for years. In 1998, his name was found on a contact list drawn up by Uwe Mundlos (so-called "Garage List"), which contained a total of 30 names linked to the ultra-right Thuringian Homeland Security (THS)—the network from which the NSU emerged.

Uwe Mundlos had direct contact with Corelli according to a report compiled by the NSU special investigator of the Bundestag, Bernd von Heintschel- Heinegg.

Corelli's name also papers in connection with a German offshoot of the Ku Klux Klan (KKK) active in the state of Baden-Württemberg. The KKK offshoot was founded by an official of the LfV with Corelli as a founding member.

A list of KKK members drawn up of the Baden-Württemberg LfV included Corelli along with two police officers who were later active later in the unit of the police officer murdered by the NSU. To date, there has been no explanation why the NSU should murder a German policewoman in broad daylight although all of the other victims were immigrant workers.

It is striking in this case that the official mugshots (which have never been made public) issued by the authorities feature persons with no resemblance to any of the NSU members. The question arises: did Corelli have knowledge about the background of the murdered policewoman and the involvement of the security forces in the far-right scene in Baden-Württemberg which could have proved embarrassing for the state?

Corelli is not the first important witness in the NSU case who has suffered a mysterious death.

Last September, another spectacular death was recorded in Stuttgart. In that case 21-year-old Florian H. burned to death in his car. On the evening of the same day, H. was due to make a statement to the Baden-Württemberg LfV on the background of the far-right milieu in the state.

Shortly after the death, the police reported that Florian H. had committed suicide. His mother disputes the police version for which there is no evidence. Witnesses had also reported that the car first exploded and only then burst into flames.

The fact that Florian H. could have been an important witness for throwing light on the NSU only became known a few weeks ago. His name appeared in a note in the final report of the investigation committee of the Bundestag NSU. It reveals that by 2012, H. had stated that along with the NSU there was also a second group of neo-Nazis, the "Neoschutzstaffel" (NSS). Representatives of both groups had met once near Heilbronn.

After Florian H., Thomas Richter, alias Corelli, is now the second important witness into the links between the NSU and the state security forces to die under dubious circumstances.