Yoon imposed martial law late Tuesday and sent armed forces to storm the National Assembly. The decree was canceled six hours later, after the parliament voted against it.
Park Se-hyun, head of the special investigation team at the Seoul High Prosecutors' Office, told reporters Yoon has been booked as a criminal suspect on insurrection and abuse of power charges, following multiple criminal complaints filed against him.
"When complaints are filed, procedurally, the subject becomes a criminal suspect," Park said.
The investigation would proceed strictly according to law and principle, "regardless of the subject's position," he added.
Insurrection is not under the jurisdiction of the prosecution.
But he said the incident involves public officials abusing their authority with the aim of disrupting the constitutional order.
Park said insurrection and abuse of power charges are "inseparable" in the case.
While sitting presidents typically have immunity from criminal prosecution, the protection does not extend to insurrection charges.
The investigation gained momentum after prosecutors detained former Defense Minister Kim Yong-hyun, a key figure in the case, following questioning early Sunday.
Kim is being held at the Seoul Eastern Detention Center on charges of insurrection and abuse of authority and will face additional interrogation.
When asked about the possibility of arresting President Yoon, Park declined to comment on future investigation plans but emphasized that "no one is above the law."
The case has sent shockwaves through South Korean politics, marking an unprecedented investigation of a sitting president for insurrection charges.
Park confirmed that the special investigation team reports directly to the Supreme Prosecutors' Office and will not brief the Justice Ministry on the investigation.
Justice Minister Park Sung-jae attended the Cabinet meeting before the martial law declaration and could become subject to investigation himself.
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