The prosecution arrested former Defense Minister Kim Yong-hyun on Sunday amid a probe into alleged treason following President Yoon Suk Yeol's short-lived declaration of martial law last week.
The special investigation headquarters probing the political turmoil said it has arrested Kim and seized his mobile phone. Kim has been sent to a detention center in eastern Seoul.
His arrest came about six hours after he showed up for an overnight questioning by the special investigation headquarters at 1:30 a.m., saying he will actively cooperate with the ongoing investigation.
Kim suggested the implementation of martial law to President Yoon Suk Yeol, who declared it late Tuesday amid an intensifying political deadlock with the opposition-controlled National Assembly. The president reversed the order six hours later after the Assembly voted to end it.