Ahn, who was dismissed on Sunday, is the second person to be taken into emergency detention after Choi was held
A snowballing political scandal moved closer to embattled South Korean President Park Geun-Hye on Thursday, with her newly nominated prime minister warning she could face a probe, hours after prosecutors detained a former presidential aide.
Ahn Jong-Beom was detained late Wednesday over suspicions that he helped Park's friend Choi Soon-Sil coerce companies into donating large sums to dubious non-profit foundations that she then used for personal gains, Yonhap said.
Ahn, who was dismissed on Sunday, is the second person to be taken into emergency detention after Choi was held Monday for questioning in the influence-peddling scandal.
Park is scrambling to deflect rising public anger over suggestions that Choi -- the daughter of a shadowy religious figure -- vetted presidential speeches, had access to classified documents, and used her influence for personal enrichment.
"Everyone including the president is equal before the law," new prime minister nominee Kim Byong-Joon declared at a press conference.
"My position is that it is (legally) possible to interrogate and investigate" a sitting president, he said.
Under South Korea's constitution, the incumbent president may not be charged with a criminal offense except for insurrection or treason.
But many argue the sitting president can be probed by prosecutors and then charged after leaving office.
Justice Minister Kim Hyun-Woong also told parliament Thursday that prosecutors could question Park, if the ongoing investigation required it.
The scandal has shaken the presidency, exposing Park to public outrage and ridicule and, with just over a year left in office, seen her approval ratings plunge into the single digits.
In an effort to deflect rising public criticism, Park had been urged to create a neutral cabinet by bringing in members from outside her ruling conservative Saenuri Party.
She has reached across the traditional political divide with a host of new appointments, including tapping the liberal Kim Byong-Joon as her new prime minister, a largely symbolic post.
She announced Thursday her pick for chief of staff, Han Gwang-Ok, a former aide to late President Kim Dae-Jung in an appointment the presidential Blue House said would help get the rattled administration back on track.
But the opposition has dismissed the reshuffle as a smokescreen.
They have called for a full investigation of Park's relationship with Choi, vowing to block the new prime minister's nomination by wielding their combined parliamentary majority.
The media has portrayed the 60-year-old Choi as a Rasputin-like figure, who wielded an unhealthy influence over Park that continued after her presidential election victory in December 2012.
Choi is the daughter of late religious leader Choi Tae-Min, who was married six times, had multiple pseudonyms and set up his own cult-like group known as the Church of Eternal Life.
He befriended a traumatised Park after the 1974 assassination of her mother -- who he said had appeared to him in a dream. Park treated him as a mentor and subsequently formed a close bond with his daughter.
Choi flew back to Seoul from Germany on Sunday to submit to herself for questioning, saying after she fought her way through a scrum of press and protesters that she had "committed a deadly sin", Yonhap reported.
Prosecutors are seeking a warrant to formally arrest Choi -- who they have deemed a flight risk and "unstable" -- before the emergency detention period expires.
The scandal comes as South Korea, Asia's fourth largest economy, faces slumping exports and high unemployment amid rising nuclear and missile threats from North Korea.