Opposition lawmakers have suggested the president had fallen "under the spell" of Choi and his daughter
South Korean prosecutors on Thursday set up a high-powered "task-force" to probe a widening scandal involving alleged influence-peddling by a close confidante of President Park Geun-Hye.
Choi Soon-Sil, an enigmatic woman with no government position, was already part of an investigation into allegations that she used her relationship with the president to strong-arm conglomerates into multi-million dollar donations to two non-profit foundations.
The scandal snowballed when it emerged that Choi had also been given advance access to presidential speeches and other documents -- a revelation that forced Park to make a televised public apology.
Prosecutor General Kim Soo-Nam told the new task force to "investigate thoroughly and reveal the whole truth", a public affairs official told AFP.
Led by the head of the powerful Seoul Central District Prosecutor's Office, the new unit will absorb the smaller team investigating the earlier allegations against Choi.
South Korean media reports have suggested Choi revised Park's speeches and may have influenced key government appointments and even the president's North Korea policy.
The scandal has been extremely damaging for Park whose approval ratings have slumped to record lows at a time of elevated military tensions with the North, and problems with skyrocketing household debt and falling exports at home.
Choi left the country in early September for Germany and, in her first interview since the scandal broke, said she was suffering from serious stress-induced health problems.
In the interview with the Segye Times, owned by the Unification Church, she admitted receiving presidential documents but denied intervening in state affairs or coercing donations from conglomerates.
"I am suffering from a nervous breakdown and I have been diagnosed with heart issues," she told the newspaper. "I could take poison and die here".
Choi is a daughter of the fifth wife of a mysterious religious figure, Choi Tae-Min, who acted as a mentor for Park Geun-Hye from the mid-1970s to his death in 1994.
Choi Tae-Min, who had seven different names and was convicted of fraud, set up a cult-like group known as Yongsaeng-gyo (Eternal Life Church), and proclaimed himself a "Maitreya" or future buddha.
Opposition lawmakers have suggested the president had fallen "under the spell" of Choi and his daughter.
Even the conservative Chosun Ilbo daily has come down on Park, with an editorial Wednesday suggesting she had "collapsed beyond recovery".
"Angry voices demanding her impeachment are flooding the street ... This is not an ordinary lame-duck phenomenon. This represents a collapse of the president's state administration," the editorial said.
Park has just over a year left in office, with presidential elections slated for December, 2017.