The US Commerce Department is deepening its probe into whether solar power companies are circumventing import tariffs, singling out some of the industry’s giants for increased scrutiny.

The agency identified eight manufacturers, including industry leaders Longi Green Energy Technology Co., Trina Solar Co. and Jinko Solar Co., for mandatory questionnaires to plumb for more information on whether they are skirting US tariffs on China-made solar products by assembling them in Southeast Asia.

The probe has already roiled the US solar industry and thrown a wrench into decarbonization plans, with companies halting projects and canceling shipments in the face of potentially hefty extra tariffs. The world’s largest solar manufacturers, mostly based in China, are diverting their focus and products to the rising European market amid the US disruptions.

Shares of major Chinese solar companies fell on Monday, with Jinko stumbling as much as 5.3%, Trina dropping 4.1% and Longi 2.5% in Shanghai.

The eight companies, located in Cambodia, Malaysia, Thailand and Vietnam, were asked to submit information by May 27, including their ownership structure and source of materials for production processes, according to letters that Commerce posted on its website. The Department has until Aug. 29 to issue preliminary findings in the circumvention case, with the extended deadline for a final determination in April 2023.

The Commerce probe also includes units of Canadian Solar Inc., Hanwha Q CELLS Co. and BYD Co., as well as New East Solar Energy Co. and Boviet Solar Technology Co. Longi, the world’s largest solar company by market capitalization, is being targeted through its Vietnamese unit Vina Solar Technology Co.