Hong Kong regulators are investigating a leading Chinese solar panel maker owned by billionaire Li Hejun after its shares took a sudden and unexplained plunge.

The Securities and Futures Commission's announcement Thursday came a day after Li was quoted in Chinese state media saying there was no investigation.

The regulator's brief statement said a "formal investigation into the affairs of Hanergy Thin Film Power Group Ltd. has been active and is continuing."

Hanergy shares had more than doubled since the start of the year but on May 20 they plunged by half in the first hour of trading before being suspended.

The stock's rise had made Li one of China's richest people with a fortune estimated at $20 billion.