Authorities have raided the main offices of national flag carrier Korean Air on Monday over suspicions of tax evasion.

Korea Customs Service on Monday said 20 investigators visited the airline's headquarters in northern Seoul from 11 a.m., where they searched offices including data management facilities. 

They also raided the offices of Hanjin Travel in central Seoul which is headed by Cho Hyun-min, the central figure in the latest "water rage" scandal and younger daughter of Korean Air’s owner, Cho Yang-ho.

This comes after the tax agency conducted last Saturday a raid on airline chairman Cho's private residence as well as Korean Air offices, in order to determine whether he or his family had imported expensive foreign items without declaring them to customs officials.

Officials have been examining flight records, overseas credit card use and tax payments of Hanjin Group Chairman Cho Yang-ho, his wife and their three adult children over the past five years.

Records on Korean Air's imported goods over the past decade are also being looked at to confirm allegations of smuggling. 

According to local media reports and tip-offs, the family brought furniture, apparel, food and other items into South Korea without paying import duties.

The Cho's are believed to have had Korean Air employees carry the products through employee passageways to avoid customs declarations.