The probe into the Sung Woan-jong bribery scandal is gaining momentum. On Tuesday and Wednesday, prosecutors questioned close aides of South Gyeongsang Gov. Hong Joon-pyo, and the governor will be summoned Friday as a suspect in the case.

Hong will be the first politician to face interrogation in the scandal, which was prompted by Sung’s allegations that he gave illicit money to eight politicians, including close associates of President Park Geun-hye.

Hong and Lee Wan-koo, who resigned as prime minister, became the first targets of prosecutors because there is strong circumstantial evidence supporting Sung’s allegations that he gave them illegal political funds.

On the day he died in an apparent suicide, Sung held a recorded telephone interview with a journalist, in which he went into details of how and when he handed over money ― 100 million won to Hong and 30 million won to Lee. Some aides to the late Sung and two politicians also came up with testimonies supporting Sung’s allegations.

Sung alleged that he gave the money to Hong for his campaign for the top post of the ruling party in 2011 and Lee for his parliamentary by-election campaign in 2013.

This means the two cases have little to do with election funds at the party level.

But a deeper issue is at stake in the Sung scandal since he claimed that he made illegal donations to past presidential election funds, including those of Park.

In a memo he left in his pocket when he committed suicide, Sung listed key members of Park’s past presidential campaigns ― former Blue House chief of staff Huh Tae-yeol, Rep. Hong Moon-jong, Incheon Mayor Yoo Jeong-bok and Busan Mayor Suh Byung-soo ― with the amounts of money beside their names. He also gave some details in the recorded interview.

Then came the statement from prosecutors that a former executive of Sung’s construction firm testified that he gave 200 million won to a member of Park’s presidential campaign team in 2012. The sum is identical to the amount Sung claimed he gave to Rep. Hong Moon-jong, then secretary-general of the party and a top manager of the campaign team.

Things like this deepen the already strong suspicion that Park’s presidential campaign was partly driven by illicit funds, including those from Sung.

When it comes to illegal election funds, the opposition should not be exempt from the prosecution’s scrutiny. Gyeonggi education superintendent Lee Jae-jung, who was a senior manager for Roh Moo-hyun’s presidential campaign in 2002, said he received 300 million won from Sung. Lee went on to say that his team asked for 200 million won, but that Sung was generous enough to add 100 million won.

It should be remembered that the prosecution found in 2004 that the Roh campaign collected a total of 11.3 billion won in illegal election funds. Even without this finding, no one would believe that in 2012, Park spent only 47.9 billion and Moon Jae-in only 48.5 billion, as they reported to the National Election Commission.

Prosecutors should get to the bottom of Sung’s allegations with the reassurance that they could investigate, if necessary, even the president and Moon.