Oh Dong-woon, chief prosecutor of the Corruption Investigation Office for High-ranking Officials (CIO), enters the CIO headquarters in Gwacheon, Gyeonggi Province, Wednesday. Yonhap

The Corruption Investigation Office for High-Ranking Officials (CIO) is encountering major difficulties in its investigation into President Yoon Suk Yeol on charges of insurrection related to his martial law imposition, as the embattled leader continues refusing to undergo questioning.

After several failed attempts to question the president over the past few days, the head of the CIO publicly urged Yoon on Wednesday to respect the judiciary's decision and cooperate with the investigation — a call which he again rejected on the day.

Yoon has repeatedly ignored the CIO's requests to appear for questioning regarding his Dec. 3 declaration of martial law, prompting the agency to resort to the alternative measure of forcibly bringing him in. It has up to 10 days to question him, before handing over the case to the prosecution next week.

"If the president has objections, they should be addressed through legal procedures within the framework of the law. However, due to his refusal to comply with the summons, the CIO has no choice but to initiate compulsory measures," Oh Dong-woon, chief prosecutor of the CIO, told reporters in front of its headquarters in Gwacheon, Gyeonggi Province, in the morning.

After his first questioning session on Jan. 15, the day he was taken into custody from his official residence, Yoon continued to defy subsequent requests for further investigation. Yoon's side claims neither the CIO nor the prosecution have the authority to investigate an insurrection charge but only the police do, according to the relevant law, while the CIO says it has the right to investigate the abuse of power charge, which is one of two key accusations facing Yoon.

In response to his persistent refusal, the CIO has ramped up pressure, considering options to either forcibly bring the president in for questioning or conduct the session at Seoul Detention Center in Uiwang, Gyeonggi Province, where he is currently being held.

A van heads toward Seoul Detention Center in Uiwang, Gyeonggi Province, Wednesday, after departing from the headquarters of the Corruption Investigation Office for High-ranking Officials (CIO). Yonhap

After Oh's remarks, a CIO vehicle was spotted entering the main gate of the detention center. But CIO officials said they were unable to question him as Yoon refused any interrogation, either at the CIO headquarters or at the center. The CIO said it had prepared an investigation room inside the center, after submitting a formal request to the center.

"We'll discuss our next plans," the agency told reporters.

However, given that Yoon's stance of refusing to cooperate with the CIO's investigation remains unchanged, the likelihood of the investigation proceeding appears very slim.

On Tuesday evening, the CIO also sent six investigators to bring him in or conduct an on-site interview at the detention center. However, they were unable to proceed as planned, as the president visited a hospital after attending a hearing on his impeachment trial at the Constitutional Court earlier in the afternoon.

The chief prosecutor of the CIO explained that the investigative authority had not been informed of Yoon's hospital visit in advance, and chose not to go to the hospital "out of respect for human dignity."

They waited at the detention center until 9 p.m., but the president returned to the center later that night after returning from the hospital. According to human rights protection regulations in the country, investigations after 9 p.m. cannot be conducted without the suspect's consent, except under special circumstances.

When asked whether such actions could be seen as an attempt to evade investigation, Oh acknowledged that the CIO considers the actions to have had that motive "to some extent."

Reporters wait outside of the headquarters of the Corruption Investigation Office for High-ranking Officials in Gwacheon, Gyeonggi Province, Wednesday. Yonhap

The case will be transferred to prosecutors once the CIO completes its investigation. For up to 20 days of custody guaranteed by the arrest warrant, the CIO and the prosecution agreed that the former uses 10 days and the latter gets the remaining 10 days.

While Oh did not provide a projected date, he emphasized that the CIO is closely cooperating with prosecutors and pledged to "handle the matter as swiftly as possible to ensure there are no procedural shortcomings."

But he said the CIO will make every effort to transfer the case before Jan. 28, which is the date the office views as its deadline before the prosecutors begin their investigation.

"The CIO will thoroughly investigate all individuals involved in the martial law incident, regardless of their rank or status," Oh said. "To ensure smooth indictments and trials, we will collaborate closely with the prosecution and other relevant agencies, fulfilling the CIO's role and responsibilities."