In the latest twist of the massive influence-peddling scandal involving former President Park Geun-hye, prosecutors are looking into allegations that the nation’s spy agency regularly made secretive money transfers to Park’s former aides when she was in office.

Former National Intelligence Service chiefs under the Park administration allegedly abused the agency’s “special activities” budget, which investigators suspect was an illicit channel for bribes for Park.

Intended to finance the most covert operations, the special activities fund is not audited and scrutinized by the National Assembly.

If proven to be true, the allegations could add new charges to Park, who has been detained in a solitary cell since she was arrested in March on a total of 18 charges including corruption and abuse of power.

On Friday, the Seoul Central District Prosecutors’ Office summoned former NIS chief Lee Byung-ho, who headed the agency from 2015-2017, for questioning.

Lee’s predecessor, Nam Jae-joon, who held the position from 2013 to 2014, was summoned earlier Wednesday over the same allegation. On Monday, Lee Byung-kee will be the third former spy chief to face grilling.

All three worked under Park and are suspected of illegally providing a total of 4 billion won ($3.6 million) to former presidential secretaries from 2013 until last year through the agency’s secret account.

Those accused of taking the NIS’ money are Ahn Bong-geun, Lee Jae-man and Jeong Ho-seong who are also under investigation.

Prosecutors suspect that the money might have been raised for a slush fund for Park.

The crackdown on the NIS came after prosecutors received testimonies by the three former presidential secretaries that Park had ordered them to regularly take illicit cash from the agency.

Part of the money is suspected to have been used to pay for Park’s beauty treatments and clothing. Prosecutors are expected to search her residence in the southern part of Seoul soon to trace the whereabouts of the 4 billion won.

The probe is among several ongoing investigations into irregularities committed under the Park and Lee Myung-bak administrations.

Prosecutors are also looking into allegations that the spy agency blacklisted media professionals deemed unfriendly to former President Lee.

The NIS Reform Committee -- a body launched by President Moon Jae-in -- also concluded that the agency masterminded a smear campaign on social media against left-wing Seoul Mayor Park Won-soon in the 2011 mayoral election.

Nam Kyoung-kook, a constitutional scholar at the University of Seoul Law School, said it is just a matter of time before prosecutors question Park Geun-hye, who is now under detention and on trial.

“A key question is whether Park was behind the scheme to amass slush funds from the NIS -- and her former presidential secretaries’ testimonies now suggest she was. But the funds are state-owned, and it would be difficult to add additional bribery charges if no favors were given to the secretaries. Her new charges could be loss of state funds and abuse of power,” Nam said.

By Bak Se-hwan (sh@heraldcorp.com)