On Wednesday, Yoon became the first sitting South Korean president to be arrested, over a probe into whether he committed insurrection when he briefly imposed martial law in early December. He is being held at the Seoul Detention Centre.
In order to hold Yoon in custody for longer, investigators at the Corruption Investigation Office for High-ranking Officials (CIO) that is leading the inquiry need to request a court to approve a detention warrant for up to 20 days.
"I think you can consider it almost finished," a CIO official told reporters, responding to a question on whether investigators were set to file a request to detain Yoon further.
The official told reporters Yoon's current arrest term was due to expire by Friday evening.
The Seoul Central District Court struck down a challenge on Thursday from Yoon's lawyers over the legality of his arrest.
The former president stonewalled efforts by the CIO to interrogate him on Thursday and again on Friday while his party has capitalised on political polarisation to improve its approval ratings since Yoon's arrest, polling showed.
"He has fully stated his basic position on the first day (of the arrest), and we believe there is no reason or need to answer the Q&A style back-and-forth," Yoon's lawyer, Seok Dong-hyeon, said in a statement.
The suspended president's legal team has denied Yoon masterminded an insurrection, a crime in South Korea punishable by life imprisonment or even technically by the death penalty.
Seok said on Friday that he expected investigators to seek a detention warrant, but hoped there would be a more careful and comprehensive consideration of the arrest's "illegality" when a court reviewed the warrant.
South Korea is grappling with its worst political crisis in decades, sparked by Yoon's brief attempt to impose martial law on Dec. 3 that stunned the nation and was swiftly voted down by parliament.
Yoon was impeached on Dec. 14 and he faces a Constitutional Court trial that started this week to decide whether to permanently suspend his powers or return him to office.
SUPPORT FOR RULING PARTY UP
Opinion polls have shown most South Koreans support his impeachment, but Yoon's legal plight and defiance over his arrest appear to have fired up some of his supporters.
The approval rating for Yoon's ruling People Power Party (PPP) was 39% in a Gallup Korea poll released on Friday, up from 34% a week ago and overtaking the main opposition Democratic Party for the first time since August which stood at 36%.
Gallup Korea said that Yoon and his party's constant messaging toward supporters appeared to have had an impact when political divisions deepened around his arrest.
Yoon was arrested after a weeks-long standoff when more than 3,000 police officers swooped on his residence. A previous attempt to arrest him on Jan. 3 failed after an hours-long standoff between investigators and Yoon's personal security.
Kim Sung-hoon, acting chief of the Presidential Security Service, was arrested on Friday for blocking investigators' initial attempt on Jan. 3 to arrest Yoon, Yonhap said.
Speaking to reporters, Kim said he was carrying out his "legitimate security duties" and denied allegations by some opposition lawmakers that Yoon ordered guards to use weapons against investigators trying to arrest him.
While key ally Washington has criticised Yoon's declaration of martial law, U.S. national security adviser Jake Sullivan warned last week there was a risk that nuclear-armed North Korea could try to exploit the political situation in South Korea.
North Korea has largely avoided public comment on the situation in Seoul, but Yoon's arrest was reported in state media on Friday, two days after the event.
The Rodong Sinmun newspaper cited foreign media to say it was the first arrest of an incumbent president in South Korea.
"Yoon Suk Yeol is not following legal procedures at the expense of the national order for individual interests," Rodong Sinmun said.
