The chairman of China’s CEFC China Energy, the private firm that has agreed to buy a nearly $10 billion stake in Russian oil major Rosneft, has been investigated for suspected economic crimes, a person with direct knowledge of the matter said.

The source said Ye Jianming was taken in for questioning this year, without saying which authorities were involved nor whether the probe was continuing. The person declined to be identified because of the sensitivity of the matter, according to CNBC.

The privately held Chinese firm, focused on energy and other investments abroad, has emerged from a role as niche fuel trader to become a rapidly growing oil and finance conglomerate, with assets across the world and an ambition to become one of China’s energy giants. It agreed in September to buy a 14.16 percent stake in Rosneft for $9.1 billion.

CEFC China Energy did not respond to requests for comment on the probe into Ye, reported earlier on Thursday by Chinese magazine Caixin without citing sources.

Shares in the firm’s listed subsidiary CEFC Anhui International Holding slumped as much as 10 percent, the maximum allowed, to their lowest since Sept. 2014, on the report.