Former South Korean Defense Minister Kim Jong-hyun was arrested today on a prosecutor’s warrant for his role in President Yoon Seok-gil’s failed attempt to impose martial law in the country, Yonhap news agency reported.

Kim, who resigned on Wednesday, is said to have played a central role in imposing martial law. According to a senior military official and opposition lawmakers who filed a motion of impeachment against the head of state, Kim had advised President Yun to declare martial law in South Korea.

The impeachment motion against President Yun was voted down in parliament on Saturday, but the head of the ruling party called on him to resign.

Kim voluntarily appeared before a Seoul prosecutor and was subsequently arrested and banned from leaving the country until the investigation is completed, Yonhap reported.

President Yun Seok-gil, former Defense Minister Kim Jong-hyun and Chief of the National Defense Staff Park Ahn-soo have been accused by the opposition of treason and South Korean authorities are investigating the case.

Yun shocked the public when he declared martial law on Tuesday night, claiming he wanted to eliminate “shameless, anti-state, North Korean-friendly forces” in his country. The president did not cite any specific threat from North Korea, which is a nuclear power, but instead focused on his domestic political rivals.

However, a majority of the South Korean parliament rejected the decree, forcing the president to lift martial law just six hours later.

Yun’s surprise decision to declare martial law has plunged South Korea – Asia’s fourth-largest economy and a key US military ally – into its worst political crisis in decades and is a “black page” in the country’s history.