Lee Jae-seung, deputy chief of the Corruption Investigation Office for High-ranking Officials (CIO), speaks during a press conference at the office's headquarters in Gwacheon, Gyeonggi Province, Thursday. Yonhap

Investigators handed over President Yoon Suk Yeol's case to the prosecution, Thursday, after the anticorruption agency repeatedly failed to get the Korean leader to answer questions related to insurrection allegations tied to last month's martial law fiasco.

That day marked 51 days since Yoon's martial law declaration and four days since a court approved a warrant for his arrest.

"We have decided to request the Seoul Central District Prosecutors' Office to proceed with its investigation in the case of President Yoon Suk Yeol, who is suspected of being the leader of the insurrection," the Corruption Investigation Office for High-ranking Officials (CIO) said.

The CIO initially planned to transfer the case on Jan. 28, believing that was their legally defined deadline, when they would have to step aside for the prosecution's investigation. However, the prosecution pushed for an earlier transfer, arguing that the deadline actually expires a day or two earlier.

The president declared martial law on Dec. 3, prompting the CIO to launch an investigation. The following day, they formally charged Yoon with insurrection.

The agency has been responsible for investigating Yoon since Dec. 8, having taken over the case after requesting its transfer from the prosecution and police.

After Yoon defied the CIO's repeated requests to appear for questioning, the agency sought and obtained a warrant to detain Yoon from the Seoul Western District Court. On Jan. 15, in a dramatic turn of events, the president was arrested during the CIO's second attempt, thereby making history as the first sitting president in Korea's constitutional history to be taken into custody.

The CIO questioned Yoon for 10 hours and 40 minutes that day, but he refused to give any statements. Yoon challenged the CIO's authority to investigate insurrection charges and claimed that his arrest was unlawful.

Since then, the CIO has requested Yoon appear for questioning five times and attempted to detain him forcibly on three occasions, but each effort was thwarted by his unwavering refusal to comply.

In the end, the CIO handed the case over to the prosecution five days earlier than originally planned, despite failing to conduct a thorough investigation into the president, who has been suspended from his duties while his impeachment trial is underway.

The referral comes amid mounting criticism that the CIO is simply wasting the window of opportunity while Yoon is in custody, as the investigation has become increasingly ineffective.

Investigators from the Corruption Investigation Office for High-ranking Officials (CIO) return empty-handed to their headquarters in Gwacheon, Gyeonggi Province, Monday, after failing to carry out compulsory apprehension procedures to forcibly bring President Yoon Suk Yeol in for questioning over his Dec. 3 martial law declaration. Yonhap

Established in January 2021, the anticorruption investigation agency was initially praised as a groundbreaking solution to address systemic abuses of power, operating independently from the prosecution.

However, criticism grew as the institution repeatedly failed to secure arrest warrants and faced multiple refusals from suspects summoned for questioning.

The CIO attempted to execute a search-and-seizure warrant on the presidential office and official residence, Wednesday, to obtain documents and evidence related to the investigation. However, the operation failed due to the presidential office's refusal to cooperate.

The prosecution is expected to apply soon to the Seoul Central District Court for an extension of the arrest warrant period for Yoon.