China on Dec. 20 launched an investigation into 17 natural gas companies, including a unit of PetroChina Co. Ltd., to see if they violated antitrust laws amid surging gas prices this winter, the South China Morning Post reported, citing the National Development and Reform Commission.

Separately, four other gas and heat suppliers were fined by the NDRC, which ordered them to return profits made from price hikes.

PetroChina did not immediately respond to an email from the South China Morning Post and its spokesman could not be reached by phone.