Park's first NIS director, Nam Jae-joon, also underwent 19 hours of questioning by prosecutors on Wednesday over the same allegations. Prosecutors plan to call in the third spy chief, Lee Byung-kee, on Monday, officials said.

The three are accused of illegally providing a total of 4 billion won (US$3.6 million) in the agency's funds to close aides to Park between 2013 and mid-2016. Prosecutors suspect the money may have been channeled into Park's secret fund.

Nam largely admitted during the questioning to sending about 50 million won to Cheong Wa Dae every month, officials said.

Prosecutors suspect that the amount of monthly payments rose to about 100 million won when Lee Byung-kee and Lee Byong-ho headed the NIS.

Prosecutors are expected to question the ex-president over the allegations.

"It's obvious an investigation of former President Park is necessary," a prosecution official said. "We will determine when and how to investigate her after taking various circumstances into consideration."

Park has been jailed at the Seoul Detention Center since March on bribery and other charges in a massive corruption scandal that led to her ouster from office. Investigators are considering visiting her at the detention center for questioning. (Yonhap)