Special prosecutors raided the state-run pension service and the health ministry's pensin division Wednesday as they started a full-fledged probe into an influence-peddling scandal involving President Park Geun-hye and her close friend.
The team led by independent counsel Park Young-soo officially began its investigation after 20 days of preparation.
The National Pension Service, South Korea's largest institutional investor, supported Samsung Group's controversial merger of two affiliates last year.
Samsung is alleged to have received the support in exchange for its donations and favors to Choi and her daughter.
Critics said the deal was intended to enhance group heir-apparent Lee Jae-yong's control of the nation's No. 1 conglomerate. His father Lee Kun-hee has been hospitalized since May 2014 after suffering a heart attack.
The investigation should be completed within 70 days and can be extended for another month upon request from the team. (Yonhap)