Two former presidential secretaries, Ahn Bong-geun and Lee Jae-man, were arrested early Friday over allegations they received at least 4 billion won ($3.58 million) from the state's spy agency, the National Intelligence Service (NIS), between 2013 and 2016 while working at Cheong Wa Dae.
Prosecutors believe NIS officials delivered about 100 million won in cash every month to Ahn and Lee, who were called the "doorknob trio" of the Park administration, along with jailed former secretary Jeong Ho-seong, for having powerful influence over state affairs.
Prosecutors also suspect they may have spent some of the 4 billion won on personal use, such as purchasing real estate.
While they have testified to receiving the money, they said they did not know the details of how it was spent.
The prosecution is expected to focus its investigation on revealing this, and is expected to inevitably turn to Park.
The prosecution is likely to expand its investigation to the ex-president after two of her closest aides admitted to receiving money from the spy agency under Park's orders.
The prosecution will focus on revealing how the money was spent, not excluding the possibility it was personally spent by Park, who is already on trial for bribery, abuse of power and other corruption charges.
If Park is found to have used the spy agency funds for personal affairs, this may adversely affect her current trial verdict.
Speculation is also rising the money was, in part, transferred to Park's close confidant Choi Soon-sil who was heavily involved both in Park's personal and state affairs.
The prosecution may later question Choi, who is being tried on charges of bribery and extortion and awaiting a court ruling slated for Nov. 15.
The prosecution is expected to first summon three former NIS chiefs ― Nam Jae-joon, Lee Byeong-ho and Lee Byung-kee ― for questioning.