WASHINGTON—Shortly after the U.S. first imposed tariffs on steel and aluminum this year, in part to crack down on oversupply from China and its closest trading partners, the Department of Commerce granted exclusions to a handful of Chinese-owned and Japanese-owned companies with U.S. subsidiaries, enabling them to continue duty-free imports of Chinese and Japanese steel.

These exclusions are the focus of a letter from Sen. Elizabeth Warren, the Massachusetts Democrat who has been one of President Trump’s most vocal adversaries and who is asking Commerce Secretary Wilbur Ross to explain why so many foreign companies with U.S. operations and so few American companies were granted exclusions.