The prosecutors had to execute two court-issued warrants so far to interrogate the defiant suspect, who billed the prosecutors as "suppressive" and "undemocratic."

It is yet unclear why Choi answered the summons. There are speculations that she willingly appeared at the probe team's office to explore how the interrogation on Park would be carried out. Choi has so far been exercising her right to remain silent.

When asked about her attitude during the questioning, the probe team's spokesman Lee Kyu-chul said Choi remained silent yet "seemed to have considerable interest in the questions," without elaborating further.

The prosecution and the presidential office are in talks over when and how to carry out the Park's inquiry, which would be the first interrogation of an incumbent South Korean president in a criminal case.

The two sides were said to have scheduled the questioning for Thursday at her office. But the probe team said it will not take place on that day after the presidential office lambasted it for leaking the timetable to a local media outlet.

Lee, the spokesman, denied the allegations that one of their investigators leaked the information, adding there is no reason for them to do so. 

President Park is currently awaiting the Constitutional Court's decision on her impeachment. She has been accused of letting her longtime friend meddle in state affairs and amass personal profits using her ties to the president.

The special counsel has been accelerating the probe into the high-profile corruption scandal as the initial investigation period is set to expire on Feb. 28. The period can be extended once, by a month, with consent from Acting President and Prime Minister Hwang Kyo-ahn.

Also on Thursday, Suh Chang-suk, CEO of Seoul National University Hospital who served as President Park's official doctor, was questioned again over suspicions he gave undue favors to a local doctor close to Choi.

Choi Kyung-hee, who led Ewha Womans University from 2014 to 2016, was also called in by investigators on the same day over suspicions she collaborated with other professors to help Choi Soon-sil's daughter gain college admission and favors in getting grades. (Yonhap)