A special counsel team investigating the December 3 martial law is accelerating its probe into former President Yoon Suk Yeol's treason charge, for which if convicted, could place him under the maximum punishment.

At a press briefing on Friday, assistant counselor Park Ji-young said investigators have made significant progress in their investigations of military officials regarding the charges of “aiding foreign aggression.”

The team suspects that Yoon and former Defense Minister Kim Yong-hyun deployed drones to North Korea last October in a bid to instigate an inter-Korean armed clash or intensify cross-border tensions to justify the decree.

Investigators questioned an aviation technology researcher from the Agency for Defense Development as witnesses on Tuesday to verify whether the drone disclosed by the North was the same type as the drone the agency delivered to the South Korean military.

The special team also intends to focus questions on the treason charge during their second interrogation of Yoon on Saturday.