President Yoon Suk Yeol delivers a public apology over last week's declaration of emergency martial law, at the presidential office in Yongsan District, Seoul, Saturday. Courtesy of presidential office

President Yoon Suk Yeol has become a suspect in the widening investigation into his short-lived martial law as he faces charges of treason and abuse of power, according to the prosecution, Sunday.

While the ruling and opposition parties clash over the impeachment motion against Yoon, the prosecution and police are moving forward with their investigations, undeterred by the outcome of the National Assembly's impeachment vote which was scrapped due to ruling party members' boycott.

During a press briefing, Park Se-hyun, head of the prosecution's special investigation team, said they have begun investigating according to due procedures, after multiple complaints were filed against Yoon.

"It is standard procedure to register an individual as a suspect when a complaint or accusation is filed," Park said.

He said the team will investigate both the treason and abuse of power accusations.

"In essence, this case involves a public official misusing authority to incite insurrection with the intent of disrupting the constitutional order. These actions constitute the criteria for treason and abuse of power under the law."

The treason charges filed against Yoon are not protected under the constitutional immunity granted to the president, allowing the investigation to proceed regardless of the outcome of Saturday's impeachment vote.

Then-Defense Minister Kim Yong-hyun attends a Cabinet meeting at the Government Complex Seoul, Tuesday. Yonhap

Ex-defense minister detained as investigations intensify

Prosecutors also detained former Defense Minister Kim Yong-hyun on charges of treason early Sunday.

He was apprehended about six hours after voluntarily appearing for questioning at the Seoul Central District Prosecutors' Office at 1:30 a.m. After the initial questioning, the prosecution placed him under detention, seized his mobile phone and searched his official residence and former office.

Kim is accused of being the mastermind behind the entire martial law saga.

Prosecutors determined that the treason charges filed against Kim constitute a serious crime and that emergency detention was necessary due to concerns over the possibility of destruction of evidence.

Kim was transferred to a detention center in eastern Seoul. Prosecutors are required to have an official arrest warrant issued by a court within 48 hours of such detention.

The prosecution also obtained a court warrant to secure Kim's phone records, specifying allegations of treason as well as mutiny under military criminal law.

Kim, a close aide to the president who attended Chungam High School in Seoul a year ahead of Yoon, is believed to have proposed the martial law declaration and played a key role in orchestrating it alongside the president.

Prosecutors are investigating his involvement in the president's declaration and subsequent lifting of martial law as well as the deployment of armed forces to the National Assembly and the National Election Commission.

Investigators from the National Office of Investigation enter the Ministry of National Defense building in Seoul, Sunday, to search and seize evidence from the office of former Defense Minister Kim Yong-hyun, who has been identified as a key figure in orchestrating the emergency martial law declaration last week. Yonhap

Investigating Kim is expected to discover the truth about the situation as spy agency officials and military commanders that participated in the short-lived martial law are giving conflicting statements about the events surrounding the martial law declaration.

Kwak Jong-geun, the former commander of the Army Special Warfare Command, stated that he had received orders from Kim to "remove members of the National Assembly." Kwak explained that he refused to follow the order, citing its illegality.

However, Lee Sang-hyun, commander of the first Airborne Special Forces Brigade, stated that he received a directive from Kwak who allegedly instructed him to "stop the vote by the lawmakers, even if it means breaking down the doors or cutting off the electricity."

The prosecution plans to first gather testimony from Kim and then cross-reference his statements with those of other individuals involved and analyze physical evidence to confirm the facts.

Separately from the prosecution, the police have set up their own investigative team of 150 officers. The police said they are not considering launching a joint investigation with the prosecution.