South Korea's disgraced ex-president, Yoon Suk Yeol, was detained for a second time Thursday over his declaration of martial law, and is being held in a solitary cell as investigators widened their insurrection probe.

Yoon plunged South Korea into a political crisis when he sought to subvert civilian rule on Dec. 3 last year, sending armed soldiers to parliament in a bid to prevent lawmakers from voting down his declaration of martial law.

He became South Korea's first sitting president to be taken into custody when he was detained in a dawn raid in January, after he spent weeks resisting arrest, using his presidential security detail to head off investigators.