The Cho family is under a series of investigations after the chairman's youngest daughter, Cho Hyun-min, a former executive of Korean Air, has been accused of allegedly assaulting ad agency officials.

The KCS' raid is the fourth of its kind conducted on the full-service carrier.

Hyun-min is the younger sister of Cho Hyun-ah, who gained global notoriety for the "nut rage" incident in 2014. She forced a plane back to the boarding gate at New York's John F. Kennedy International Airport because she was upset with the way her nuts were served -- in an unopened bag instead of on a plate.

In March, Hyun-ah made a comeback as the head of KAL Hotel Network, the operator of the Grand Hyatt Incheon, near Incheon International Airport.

Both sisters have since been stripped of their company posts.

Meanwhile, the chief of the country's customs office said earlier that Chairman Cho and his only son, Won-tae, president of Korean Air Lines, will be questioned over allegations of smuggling.

Earlier, the customs office said Hyun-min, her older sister Hyun-ah and their mother are on the list for questioning.

The Korea Customs Service has been investigating allegations that the founding family members have smuggled luxury goods into the country without properly paying duties.

Smuggling is an offense that carries a maximum sentence of five years in prison and fines up to 10 times the duties evaded. (Yonhap)