The investigation into the spy agency’s alleged political intervention took a new twist when the prosecution discharged the case’s chief investigator on Friday. His dismissal followed the arrest of three officials from the National Intelligence Service on charges of smearing the opposition presidential candidate on Twitter last year.

The prosecution said it dismissed the prosecutor heading a special investigation team from his post because he did not follow the standard operating procedure when he arrested the NIS officials. Specifically, the chief investigator did not obtain approval from the senior prosecutor in charge when he sought search, seizure and arrest warrants. Nor did he abide by a statutory rule requiring him to notify the NIS director of an investigation into a case involving NIS agents immediately when he launched it.

It was far from thinkable for the chief investigator working on the high-profile case to take action without prior consultations with higher-ups. Equally unimaginable was the prosecution’s decision to immediately remove him from the investigation.

What is certain, however, is that the chief investigator did not trust the top brass when he decided to ignore the standard operating procedure. Instead, he may have believed they would thwart his investigation into the spy agency’s alleged online smear campaign if he reported his plan to widen the investigation and arrest the three NIS officials. Their alleged use of Twitter in the campaign against the opposition candidate was a new finding from the investigation that had started several months before.

But a news report claimed that he did report his plan to a senior prosecutor in the line of command, and that he decided to take action on his own when the senior prosecutor continued to sit on his proposal to arrest the NIS officials. The chief investigator put his career as a prosecutor on the line when he decided to arrest the officials without obtaining official approval. If past experience is any guide, it seems to be a matter of time before he is pressured to resign as a prosecutor.

The conflict between the prosecution’s leadership and the chief of the special investigation team followed the recent forcible resignation of Chae Dong-wook from the post of prosecutor-general. The main opposition Democratic Party and other critics believe that President Park Geun-hye’s administration forced Chae out of office in an attempt to scale down the prosecution’s investigation into an allegation that former NIS director Won Sei-hoon ordered the smear campaign. Chae resigned when the Justice Ministry, allegedly in collaboration with the presidential office, launched an inquiry into an allegation that he fathered a son in through an extramarital affair more than 10 years ago.

When the chief investigator was discharged, the Democratic Party was quick to denounce the prosecution’s top brass for exercising undue influence on the ongoing investigation. The party also promised to push for the dismissal of the justice minister, who it believed was behind the prosecution’s action against the chief investigator this time again.

What the prosecution needs to do under these circumstances is to commit itself to exercising no undue influence on the special investigation team and make good on their promise. Its image will be tainted again if it allows itself to be bandied about by the power elite as it did previously.