South Korea’s special counsel probing alleged obstruction of the National Assembly’s martial law vote said Wednesday it has asked a court to secure testimony from former People Power Party leader Han Dong-hoon.

Prosecutors said Han, a reference witness, refused to cooperate despite being “indispensable” to the investigation, prompting the rare request for pretrial questioning.

The panel is examining whether then floor leader Choo Kyung-ho altered meeting locations to prevent lawmakers from voting on ending martial law at the request of former president Yoon Suk Yeol.

Investigators believe Han exchanged messages with Choo during the crisis, possibly coordinating their responses.

Special counsel officials stressed they will seek similar court measures against any other witnesses who clearly refuse to appear.

Han has recently published a book and given interviews touching on the events, which prosecutors say underscores the need for his testimony.