Special prosecutors on Sunday sought an arrest warrant on a presidential aide who has allegedly been involved in assisting in the inappropriate medical treatment of President Park Geun-hye.
Lee Young-seon has been detained since late last week for helping doctors close to Park's jailed confidante Choi Soon-sil treat the chief executive without going through security checks.
He is also accused of opening a mobile phone under an alias that was used by officials in the presidential office of Cheong Wa Dae and for committing perjury during a parliamentary hearing.
The Independent Counsel Park Young-soo and his team cited Lee breaking the country's medical practice law, as well as those governing telecommunication rules that ban setting up mobile devices under an alias.
The request for the arrest comes just two days before investigators have to wrap up their probe.
The special counsel has asked for an extension of its investigation for a month, but the acting president, who has the final say on the matter, has yet to make a reply.
The arrest warrant is expected to be checked by the court Monday.
Related to the investigation closely linked to President Park's impeachment, formal trials for a senior aide to the chief executive and a Cabinet minister over the creation of a cultural blacklist are expected to start this week.
The trial is centered on actions taken by former presidential chief of staff Kim Ki-choon and ex-Culture Minister Cho Yoon-sun, who are under suspicion for creating the list to restrict artists critical of the government from getting state support.