Yim Feng, Chairman and Chief Executive Officer of Guotai Junan Securities Co. (File photo)

One of China's biggest securities firms, Guotai Junan Securities Co. has announced the decision on Wednesday to return its Chairman and Chief Executive Officer, Yim Feng to active duty.

In a statement issued on Wednesday, the company said Yim had been assisting investigations by mainland authorities in a personal capacity.

It said neither Yim nor the company was the target of the investigations.

The brokerage appointed provisional replacements last month after losing contacts with the 52-year-old Yim for several days.

The shares of company then suffered a 12 percent sharp slide.

Some media outlets speculated the investigation was related to Yao Gang, former vice chairman of China's Securities Regulatory Commission, who was suspected of "serious violation of discipline".