JOHANNESBURG (Reuters) - South African prosecutor Gerrie Nel, who laid out the murder case against Paralympian Oscar Pistorius, has resigned to join a civic group that plans to bring private prosecutions in criminal cases that the state opts not to pursue.

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Nel, nicknamed “The Bulldog”, eventually secured a six-year jail term for the 2013 murder of the athlete’s girlfriend Reeva Steenkamp, after an earlier manslaughter verdict was upgraded on appeal. The case remains open after he challenged the new sentence on the grounds it was still too lenient.

National Prosecuting Authority (NPA) spokesman Luvuyo Mfaku, confirming Nel’s resignation, said the file could now be assigned to another lead prosecutor.

At the trial, Nel painstakingly crafted a picture of Pistorius as a cold-blooded killer. The athlete known as the Blade Runner argued he shot dead Steenkamp by accident, firing through a bathroom door after mistaking her for an intruder in his house.

Nel is joining the AfriForum group, whose chief executive Kallie Kriel said it aims to bring private prosecutions of cases in the public interest. He said public figures could be called to account in cases that are dropped by state prosecutors.

“We will take over where the state fails in their duty,” Kriel told Reuters. “We believe nobody in the country is above the law, including the president.”

Asked if he planned to take on cases involving President Jacob Zuma, Nel told a news conference: “We will not identify any subject of an investigation at this stage.”

Zuma’s popularity has been jolted by a series of scandals, which last year contributed to the ruling party’s worst electoral performance since the end of white minority rule in 1994.

Nel, in a 36-year career at the NPA, was known for taking on the powerful and politically connected. His biggest scalp was former police chief and Interpol head Jackie Selebi, who was convicted of corruption in 2010.

Some on social media said Nel’s move to the private sphere was a mistake. Lawson Naidoo, the Executive Secretary of the Council for the Advancement of the South African Constitution, tweeted that he “runs the risk of destroying his legacy of a proud career in public service.”

Responding to the criticism, Nel said: “I understand all the perceptions. I won’t sell my integrity. I’m just excited to be part of something new.”