South Korean President Yoon Suk Yeol narrowly avoided impeachment on Saturday after the motion was scrapped for failing to meet the quorum of two-thirds of the 300-member Assembly.

However, his legal troubles are far from over. Police, prosecutors and corruption authorities have all begun wide-ranging investigations into his controversial declaration of martial law and opposition parties vow to pursue alternative avenues of accountability.

The prosecution said Sunday the president has been booked as a suspect on charges of treason.

"Numerous complaints have been filed, and an investigation is currently underway as part of the standard process," said Park Se-hyun, head of the special investigation team probing President Yoon’s brief martial law declaration last week. Park added that prosecutors are investigating charges of treason and abuse of power against Yoon.

Treason, or attempted treason, is one of only two offenses a sitting president can be indicted for. If the treason charges against Yoon stand, he could be the first South Korean president to be indicted while in office.

Potential legal accountability

While impeachment was designed to address political misconduct, Yoon may still face civil or criminal charges even after his term ends, when presidential immunity from prosecution no longer applies. Legal scholars have flagged Article 77 of the Constitution, which restricts martial law to extraordinary national emergencies such as war or armed rebellion, as possible grounds for legal action against him.

Some experts have also raised the prospect of treason charges if evidence emerges that the martial law declaration was intended to suppress political opponents or restrict legislative functions.

The president declaring martial law may not in itself amount to treason, but what followed, such as soldiers' attempt to occupy the National Assembly and arrest political leaders could, the former chief of National Assembly Research Service Kim Man-Heum told The Korea Herald.

"However, it could be controversial once the president claims that he was proceeding according to legitimate procedures, saying that he followed the decisions of the National Assembly once they passed to cease the martial law," he added.

If not treason, Yoon may still face charges of abuse of power, which is an impeachable offense.

The prosecutorial and investigative arms of South Korea have ramped up their probes into President Yoon Suk Yeol over his Dec. 3 martial law declaration, seeking to pursue charges of insurrection and abuse of power, among others.

Adding to the intensity, speculation is growing that a permanent special prosecutor proposed by the Democratic Party of Korea could soon join the fray. If activated, this would bring the number of simultaneous investigations to four, making it one of the most comprehensive legal probes in South Korean history.

The Supreme Prosecutors' Office established a special investigative headquarters, headed by Seoul High Prosecutors Office Chief Park Se-hyun. The task force includes 20 prosecutors and over 30 investigators. In a significant move, military prosecutors have joined the team to probe the involvement of military officials in the decision-making process.

The task force is expected to carry out search and seizure operations once it establishes the basic facts. These efforts aim to clarify whether the martial law declaration bypassed necessary constitutional and legal procedures.

The National Police Agency has also formed a dedicated team of 120 officers to look into four separate cases, including allegations of insurrection, rebellion and abuse of power.

Meanwhile, the Corruption Investigation Office for High-Ranking Officials has assigned its 4th Investigation Division to examine related allegations of abuse of power. Although insurrection charges fall outside the CIO’s jurisdiction, the agency is investigating other potential crimes tied to the crisis.

The Democratic Party has proposed activating a permanent special prosecutor to ensure impartiality and prevent jurisdictional conflicts among investigative bodies. The proposal is set for a vote in the National Assembly on Dec. 10. If passed, the special prosecutor will operate independently of the presidential office, further heightening pressure on Yoon.

While Yoon’s supporters see the failed impeachment as a victory, critics argue that the investigations will hold him accountable. The outcome of these inquiries could significantly shape Yoon's legacy and redefine executive accountability in South Korea’s democracy, experts say.