There are many reasons the state prosecution, set to reopen its investigation on a sex bribery scandal involving a former vice justice minister, should leave no stone unturned. As President Moon Jae-in said, it should do everything it can to find out the truth.

Overall, the case seems very complicated, with legal and political factors entangled. Depending on how the prosecution’s investigation goes, the issue could batter politics heavily as it already pits the ruling party against the main opposition party.

But the core issue in this case is rather simple: Former Vice Justice Minister Kim Hak-ui is suspected of having received bribes, including sex, from a businessman. There are also allegations that some people in powerful positions -- including those at Cheong Wa Dae and the prosecution -- interfered with the investigation in order to protect Kim.

The sex bribery scandal, which took place more than a decade ago, broke in 2013 when Kim -- then a senior prosecutor -- was appointed as vice justice minister by former President Park Geun-hye.

The announcement was met with allegations that Kim had kept corrupt ties with a real estate businessman, Yoon Jung-cheon, since 2005 and received bribes, including sexual services in return for business favors.

Yoon allegedly invited Kim and other influential people to sex parties at a villa in Wonju, Gangwon Province, and other places in 2007 and 2008. There was video footage in which Kim was said to have been caught participating in one such sex party. He denied the allegations, but had to resign six days after his appointment as controversy raged over the video clip.

The police and the prosecution investigated the allegations twice -- in 2013 and 2014 -- only to clear Kim of all charges. But there has been consistent suspicion that the investigations were tampered with.

In fact, there have been testimonies, including those from women who claimed they participated in the sex parties hosted by Yoon, but investigators ignored their accounts of what transpired.

There were also allegations of outside interference with the investigation. For instance, some police officers claimed that Cheong Wa Dae ignored their report on the sex video involving Kim and pushed ahead with his appointment as vice justice minister.

It is also alleged that police investigators who worked on the case were transferred or demoted. One incomprehensible fact is that the prosecution rejected police requests for arrest and search and seizure warrants against Kim.

Recently, a woman who claims to have participated in the sex parties alleged in a television interview that she was sexually assaulted by Kim. National police chief Min Gap-ryong also testified in the National Assembly that Kim is clearly visible in a video clip of Yoon’s sex parties.

So it came as no surprise that a Justice Ministry truth panel, which had already been reviewing the Kim case, called on the prosecution to reopen the probe.

The problem is that the forthcoming investigation itself has become a hot political issue that pits the ruling party against opposition parties. In part, the truth panel set the stage for the political strife since it recommended “selective” investigation of former officials at Cheong Wa Dae.

One pinpointed by the truth panel is Rep. Kwak Sang-do of the main opposition Liberty Korea Party, who was the senior presidential secretary for civil affairs in 2013. The truth panel said he could have interfered with the investigation to protect Kim.

In contrast, the panel excluded Rep. Cho Eung-cheon of the ruling Democratic Party of Korea although he was one of Kwak’s closest aides during the first few years of the Park administration.

It is apparent that the Justice Ministry truth panel was swayed by partisan considerations, targeting only the opposition members. The ruling party is also trying to tie the Kim case to Liberty Korea Party leader Hwang Kyo-ahn, who was the justice minister at the time.

Partisan politics is the last thing the prosecution should consider as it reopens the case. It ought not repeat the misdeeds it committed in 2013 and 2014.