The move came two weeks after a Seoul court denied the prosecution's request for a warrant to arrest Lee over multiple assault charges.
In addition to the assault charges, the 69-year-old Lee is accused of hiring some two dozen maids from the Southeast Asian country for at least the past decade by sponsoring them with traineeship visas in violation of the law.
The authorities suspect Lee had the airline's human resources department and its Manila branch recruit the housekeepers. Korean Air officials from the two offices have also been questioned by the immigration investigators.
Lee, mother of the former Korean Air vice president who became infamous for the "nut rage" incident, has also been under investigation over allegations that she took advantage of her status to routinely assault and insult people physically and verbally. (Yonhap)