Investigators from the National Investigation Office of the National Police Agency leave the presidential office in central Seoul, Wednesday, after failing to raid the office to collect evidence related to President Yoon Suk Yeol's martial law fiasco. The office did not allow their entry and instead submitted a small amount of material. Yonhap

Investigators are closing in on President Yoon Suk Yeol over his short-lived martial law, with Yoon being called a "suspect" in a search warrant and identified as "the leader" of an insurrection.

A team of 18 investigators from the National Investigation Office (NIO) of the National Police Agency (NPA) arrived at the presidential office in central Seoul on Wednesday to search for and seize evidence related to Yoon's surprise Dec. 3 martial law declaration.

The president's office, the Cabinet meeting room and the Presidential Security Service were the subjects of the operation. The investigators said they sought to secure evidence, including attendance lists and minutes of Cabinet meetings held in the lead-up to and after Yoon's martial law declaration, as holding a meeting is a requirement for the president to impose and annul martial law.

Their search warrant identified Yoon as the suspect.

However, they were unable to execute the warrant as security personnel blocked their way. According to the relevant law, to enter and search security facilities, including the presidential office, investigators are required to get agreement with those in charge of facility management. Conducting a forcible search is barred. After nearly eight hours of confrontation, investigators obtained only a small amount of material submitted voluntarily by the security service.

"The presidential office submitted a letter of explanation stating why it could not accept the warrant. There will be no additional search and seizure," the NIO said.

The president was not at the office during the entry attempt, according to reports.

Besides the presidential office, the NIO conducted search-and-seizures at the NPA, the Seoul Metropolitan Police Agency and National Assembly Police Guards. The NPA was subject to a search as police were involved in preventing lawmakers from entering the Assembly compound on the night of the martial law declaration.

The Corruption Investigation Office for High-ranking Officials (CIO), which is also probing the case, said it has "enough will" to apprehend Yoon depending on its findings.

"We are working hard on the investigation. We will review the legal requirements whether to apprehend the president or seek an official arrest warrant," Oh Dong-woon, the chief prosecutor at the CIO, said during a meeting of the Assembly's Legislation and Judiciary Committee, Wednesday.

He added that "it is the principle of investigation to arrest the prime suspect" in a treason case.

Oh Dong-woon, right, chief of the Corruption Investigation Office for High-ranking Officials, speaks during the National Assembly's Legislation and Judiciary Committee meeting, Wednesday. Yonhap

This suggests that Yoon may become Korea's first sitting president to be apprehended or arrested while in office. Under the law, a president is immune from prosecution while in office, except in cases of treason.

With Yoon as an official suspect, the investigation into him as well as top police and military officers is set to pick up pace. If found guilty of treason, he could face the death penalty or life in prison.

So far, all relevant investigative authorities — the CIO, the NIO and prosecutors — have been handling the case on their own. On Wednesday afternoon, the CIO, police and the defense ministry launched a joint investigation unit over Yoon's treason charges.

Yoon is already under an overseas travel ban as police and prosecutors investigate whether he and his associates committed treason when they sent armed troops into the National Assembly last week.

Earlier in the day, the Seoul Central District Court issued an arrest warrant for former Defense Minister Kim Yong-hyun on charges of treason and abuse of power. Kim has been accused of recommending martial law to Yoon and sending troops to the Assembly to block lawmakers from voting on lifting it.

In the arrest warrant, the prosecution stated he "incited rebellion in collusion with President Yoon Suk Yeol," indicating he was a major figure working for insurrection and Yoon was the prime figure responsible for it.

The Seoul Dongbu Detention Center / Yonhap

Kim is the first person to be formally arrested over the martial law decree. By law, those who participate in plotting treason could be punished by death, life imprisonment or a prison term of at least five years.

Earlier in the day, Kim tried to take his own life at a detention facility in Seoul, where he is being held under emergency detention.

"The suspect (Kim) tried to take his own life, but the staff rushed there and forced the door open. He immediately gave up on his attempt. He was later moved to a secure cell and his health remains stable," Shin Yong-hae, commissioner-general of the Korea Correctional Service, said during the judiciary committee meeting.