Former President Lee Myung-bak has denounced the prosecution’s investigation of his aides as “political retribution,” while President Moon Jae-in is infuriated by the description.

The unusual head-on collision between the incumbent and former presidents is regrettable.

Lee’s claim is arguable, but it is not right to try to turn the investigation into a political mud fight without clearing related suspicions.

Not only Lee and his close associates but also Moon and ruling party lawmakers must refrain from using language that could encourage partisanship.

Most importantly, investigation must be done fair and square.

“I cannot bottle up anger at former President Lee mentioning political retribution for the death of former President Roh Moo-hyun,” Moon said Thursday, “Lee’s description of Cheong Wa Dae as if it has pulled strings behind the prosecution to take political revenge is an insult to the government and a repudiation of judicial systems. It passes the bounds of political magnanimity.”

This is Moon’s refutation of Lee’s statement made a day earlier that the prosecution’s investigation under Moon’s drive to uproot irregularities of the past is political maneuvering to wipe out conservatives and a political vendetta over the death of Roh, whom Moon served as chief of staff.

It seems that Moon was offended the most by Lee’s association of Roh’s death with the investigation.

It is undeniable that the prosecution has tried to dig up dirt on only the past two conservative regimes, which cannot but arouse suspicions of political retribution.

The opposition Liberty Korea Party criticized Moon’s fury, saying it could hamper the independence of the investigation. The party has called for the investigation of the secretive “special activities” budget of the National Intelligence Service under former liberal presidents including Roh.

The ruling Democratic Party of Korea alleged that part of the money the NIS is suspected of offering secretly to Lee’s close aides when he occupied Cheong Wa Dae was spent buying luxury goods for his wife.

This is suggestive of a luxury wristwatch allegedly offered as a bribe to Roh’s wife. Allegations lacking supporting evidence will only thicken suspicions of tit-for-tat retribution.

Moon’s expression of strong emotions regarding a case currently under investigation is politically incorrect and undesirable, even considering his close ties with Roh, as the investigation could become biased.

It is incontestable that a former president should be investigated if it is necessary to clear related suspicions.

Political retribution should have no place in investigations, but assertion of its existence without trying to clarify suspicions is less than convincing.

If Lee intends to prove his innocence or to take responsibility as he said in the statement, “the buck stops with me,” the first thing he should have done was either to present evidence or to answer questions directly.

It is also inappropriate for his aides to say that they have some revealing files that were made during the Roh administration, as this sounds like a threat.

However, it is not only Lee and his aides but also Moon and his partisans that must refrain from using emotionally charged words that could influence the investigation.

The point is whether the prosecution is able to investigate suspicions fairly. It must not take cues from Cheong Wa Dae regarding the investigation. Prosecutor General Moon Moo-il vowed to keep following legal procedures. Fundamental as this may sound, it is the right direction. Summoning and questioning those involved and then seeking punishment for violation of laws are basic steps that must be taken, whoever is investigated.

To keep the nation united, both Lee and Moon should avoid making a political football of the investigation. And they must also refrain from offending each other.

The prosecution ought to do its job cool headedly and without bias. Unless it breaks the long-established public view that it has served whatever president was in power, the investigation will boomerang years later. It is about time to put a stop to controversies over political retribution arising each time a regime changes.