One of the most important neo-Nazi trials gripped the country for four years is about to come to an end.

This Tuesday the Federal prosecutor demanded a life sentence for Beate Zschäpe, the only surviving member of the NSU terror cell.

What was the NSU trial about?

The far-right terror cell known as the National Socialist Underground (NSU) allegedly murdered 10 people, carried out two bombings and robbed banks in various German states between 2000 and 2007. The NSU consisted mainly of three people - Uwe Böhnhardt, Uwe Mundlos (both dead) and Beate Zschäpe. The trio came from the eastern German city of Jena, where they were active in the right-wing extremist scene.

Most of the murder victims had an immigrant background, the only exception being a German policewoman who was gunned down in 2007. The NSU is also said to be responsible for the nail bomb attack that left 22 people injured in a Turkish neighborhood in Cologne in June 2004. Investigators initially attributed the murder attempt to the local Turkish gambling mafia. Years passed before authorities shifted their suspicion to the right-wing terrorists. In the early years of the investigation, family members of the victims faced allegations that their husbands, sons and brothers had been involved in criminal activities, thus making them targets for criminal groups.

Read: How Germany remembers the NSU nail bomb attack

The NSU trial began on May 6, 2013 in the 6th Criminal Division of Munich's Higher Regional Court. Beate Zschäpe and four suspected accomplices are being tried. Zschäpe has been charged with complicity in 10 counts of murder, arson, the formation of a terrorist organization and membership in a terrorist organization. She and co-defendant Ralf Wohlleben, a former official of the right-wing extremist National Democratic Party of Germany (NPD), have been held in custody since 2011.

The NSU proceedings began in 2013 - Zschäpe and four suspected accomplices are on trial

Who were the NSU members?

In the mid-1990s, the trio was active in the "Thüringer Heimatschutz" neo-Nazi organization, based in the eastern German state of Thuringia. Uwe Böhnhardt and Uwe Mundlos took their lives in November 2011 to avoid being arrested after botching the latest in a series of bank robberies. After their deaths, Zschäpe made a video confession before setting the trio's home on fire. She turned herself in to the authorities four days later.

Böhnhardt had already managed to avoid capture before: In January 1998 Böhnhardt’s garage in Jena was searched by police after a tip-off. They found enough explosives there to cause a major explosion, but Böhnhardt managed to flee the city before an arrest warrant was issued.

Between 1998 and November 2011, Zschäpe, the main NSU trial defendant, lived a clandestine life with Mundlos and Böhnhardt. Nationwide, 160 police officers were tasked with the murder investigation. The NSU trio, however, remained unknown. Only after Zschäpe's video confession did the relationship between the crimes begin to materialize.

Among other things, police found a gun in the burned-out apartment that was the one used in nine of the murders, giving them concrete evidence of the NSU's involvement.

Mundlos and Böhnhardt committed suicide in 2011, Zschäpe turned herself in to authorities four days later

The relatives of the NSU murder victims hoped Zschäpe would explain why their family members were killed

What role did the government play?

The Bundestag, Germany's lower house of parliament, and many state parliaments set up investigative committees to shed light on the security authorities' failures in the NSU case. In the Bundestag's inquiry, members of parliament accused the prosecutor in charge, Herbert Diemer, of not having investigated evidence in order to protect domestic secret service agents and informants who may have had prior knowledge of crimes.

One major unresolved mystery remains that of Andreas T., an intelligence officer for the state of Hesse who was sitting in the internet cafe in Kassel on April 6, 2006, when the owner, Halit Yozgat, was shot dead. He at first failed to come forward and, when his presence there was disclosed, he claimed to be unaware that the shooting had taken place. His case awakened further fears that German intelligence agencies may have in some way colluded with the NSU.

Read: NSU: News from the informant front

The role of informants, who were often high-ranking figures in the neo-Nazi scene, has also come under particular scrutiny. Several of them received large sums of money from the state, some of which actually went toward supporting the neo-Nazi scene, and they were also often warned in advance of house searches.

It also remains unclear to what extent German intelligence authorities acted on the information they provided, or whether this information was really useful.

Zschäpe's defense team will present their closing arguments in September, a verdict could come later this year

Other criticism leveled at the investigation of the NSU's crimes, apart from allegations of systemic racism on the part of German authorities, revolved around the lack of cooperation between the various intelligence agencies and state interior ministries, which are responsible for police in the respective states. More than 30 official bodies in all were involved in the probe, often getting in each other's way owing to local loyalties rather than working together.

Read: Neo-Nazi NSU murder report shows Angela Merkel's 'broken promises'

It also emerged that shortly after Zschäpe's arrest, an official at Germany's domestic intelligence service, the BfV, had shredded several files pertaining to informants involved with the NSU. Other state agencies followed suit by destroying some 400 files and documents connected with the case. Officials said at the time that the files were destroyed to protect informants and stop state secrets from becoming public.

The victims' survivors and their attorneys pressed intelligence agencies over their questionable role in the affair but failed to get answers.

Guilty verdict

Beate Zschäpe was found guilty and sentenced to life in prison in 2018. The court has heard 815 witnesses and interviewed 42 experts.

The verdict is being challenged from both sides — and questions about the involvement of informants as well as potential cover-ups remain the subject of heated debate.