Kim was formally arrested early Wednesday over allegations of conspiring with President Yoon Suk Yeol in imposing the emergency decree last Tuesday. He had been in custody at the Seoul Eastern Detention Center since Sunday.
Kim tried to hang himself using improvised bindings made from his underwear in a bathroom at 11:52 p.m. Tuesday while awaiting the court's decision on prosecutors' request for his arrest warrant, Shin Yong-hae, commissioner general of Korea Correctional Service, said.
Detention officials discovered and immediately stopped the attempt, Shin told a parliamentary committee.
"He abandoned the attempt when officers opened the door. He is currently in a protective cell and shows no health concerns," he added.
The Seoul Central District Court issued the arrest warrant, citing the gravity of the charges against Kim and the risk that he could destroy evidence.
He faces charges of insurrection and abuse of power.
Kim is the first person arrested in connection with the incident. Prosecutors believe he colluded with Yoon and played a key role in the failed martial law declaration.
They suspect Kim proposed the plan to Yoon and mobilized soldiers to storm the National Assembly and the National Election Commission's headquarters in Gwacheon, south of Seoul.
With Kim's arrest, the investigation is expected to focus on Yoon, who is currently barred from leaving the country.
Under South Korean law, a sitting president is immune from prosecution except in cases of insurrection or treason.
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