One of the companies is Bitmain Technologies Ltd., a China-based firm that is the world’s leading manufacturer of cryptocurrency mining machines. The other, Cango Inc., is a potential acquisition target for Bitmain, Representative Zachary Nunn of Iowa wrote.

Bitmain and Cango “appear to be scaling operations in the U.S. through complex ownership structures and financing arrangements that may not be fully transparent to regulators or the public,” Nunn wrote.

Bitmain, based in Beijing, supplies about 82% of the world’s cryptocurrency mining machines, according to an April report from the University of Cambridge Judge Business School. Last week, Eric Trump, one of the president’s sons, took public a Bitcoin mining company that’s heavily dependent on Bitmain equipment. A few weeks earlier, the company, American Bitcoin Corp., bought about $314 million worth of machines from a Bitmain subsidiary. Representatives of American Bitcoin didn’t respond to requests for comment.