The U.S. Commerce Department said it would investigate whether Chinese companies sell solar panels in the United States at unfair discounts and receive illegal government subsidies. The trade spat, one of several sensitive economic and trade issues between the United States and China, could lead to steep duties on imports of Chinese panels and help struggling domestic manufacturers. The action is opposed by companies in the U.S. solar industry that count on importing cheap panels to boost solar power generation. It comes as the administration of President Obama faces criticism from Republicans in Congress about domestic aid to solar and other renewable energy companies. The Commerce Department said Wednesday that it had accepted a petition from SolarWorld Industries Americas Inc. Last month, the company asked the U.S. government to slap duties on Chinese solar cells and modules.