Prosecutors on Friday raided the headquarters and offices of Woongjin Group as part of their investigation into allegations that several executives of the troubled mid-size conglomerate fraudulently issued commercial papers (CPs).

The probe comes after the nation's securities regulator had reported the group's chairman Yoon Seok-keum, and four other executives to the Seoul Central Prosecutors' Office for alleged unfair trading in early May.

A team of investigators and prosecutors seized relevant documents and computer discs during the raid of the group's headquarters in central Seoul and five other offices, officials said.

Prosecutors also raided the residence of chairman Yoon as well as houses of other two executives, they added.

After analyzing the seized documents, prosecutors said they will soon call in Woongjin executives to question them over whether there were irregularities in the process of issuing CPs.

The five executive are accused of issuing CPs worth around 100 billion won ($91 million) under the name of the group last July even with prior knowledge that the group had lost its ability to pay back its debt in the aftermath of a credit rating downgrade, according to the Securities and Futures Commission, an arm of the Financial Services Commission that looked into the case.

In September, they issued more CPs worth 19.8 billion won without publicizing the fact that the group had applied for management normalization of Woongjin Holdings with the court after scrapping its earlier plan to dispose of Woongjin Coway, the commission added.

Woongjin Chemical's parent Woongjin Holdings has been placed under court receivership since last October due to financial difficulties. (Yonhap News)