Prosecutors on Wednesday called in aides of two ruling party heavyweights for questioning in search of a breakthrough in a graft probe whose neutrality is being criticized.

They called in the secretaries of former Prime Minister Lee Wan-koo and South Gyeongsang Province Gov. Hong Joon-pyo to gather information on their whereabouts when deceased businessman Sung Woan-jong claimed to have given money to them.

Lee, who resigned upon mounting pressure last week, and Hong are two of the eight politicians on the list left by Sung before his apparent suicide earlier this month, suggesting he had given illicit political funds to them.

The investigation, slowed by a lack of evidence, appeared to hit another snag Tuesday following a message from President Park Geun-hye calling for an investigation into her predecessors’ special pardon on Sung.

Her remarks, given through her chief press secretary, came under fire from opposition and local observers as an attempt to sway the investigation. Sources from the prosecution were quoted as saying in reports that Park’s message tightened their discretion as any investigation outcome may appear to be directed by Cheong Wa Dae.

They also cited current laws that make it impossible to investigate a past pardon decision.

Under the law, a presidential pardon is an act of the state that is not legally binding. If it is suspected of having a criminal motive ― such as the president or his or her confidants receiving money or favors in exchange for the pardon ― it can be subject to investigation. But in such cases, investigators must prove that the money or favors were provided in exchange of the promise of the pardon.

Rep. Jun Byung-hun of the main opposition New Political Alliance for Democracy claimed Park’s argument lacked substance as there was no evidence suggesting a deal was made for Sung’s pardon.

The NPAD has tried to shift the blame over Sung’s pardon to the Saenuri Party, claiming that the businessman was pardoned in early 2008 at the request of then-President-elect Lee Myung-bak.

The Saenuri Party dismissed the argument and demanded NPAD leader Moon Jae-in ― who was chief of staff to then-President Roh Moo-hyun ― to explain exactly why Sung was pardoned.

Yang Seung-ham, a politics professor at Yonsei University, pointed out that because the incident happened several years ago and Roh no longer alive, it is very unlikely that either side will be able to provide enough evidence to prove their case.

Contrary to Park’s emphasis on rooting out corruption, observers said Park’s public address was designed to raise support ahead of Wednesday’s by-election while dodging responsibility for the graft scandal. The eight suspected of receiving money from Sung consist of her former and incumbent chief secretaries and ruling party members.

“Park appears to be trying to ‘water down’ suspicions against her and her staff with political intent for the by-election,” Yang said.

Justice Minister Hwang Kyo-ahn, meanwhile, underscored that the investigation would stay neutral.

With regard to any probe into the pardons, Hwang said he was looking into all possibilities, but added that an investigation was unlikely if evidence of wrongdoing did not turn up.

“I’m just saying that we are not confining the scope of the investigation to any specific party,” he told the parliamentary legislation and judiciary committee.

By Yoon Min-sik (minsikyoon@heraldcorp.com)