DuPont Co. suspected its onetime partner in China was getting hold of its prized chemical technology, and spent more than a year fighting in arbitration trying to make it stop.

Then, 20 investigators from China’s antitrust authority showed up.

For four days this past December, they fanned out through DuPont’s Shanghai offices, demanding passwords to the company’s world-wide research network, say people briefed on the raid. Investigators printed documents, seized computers and intimidated employees, accompanying some to the bathroom.

Beijing leans on an array of levers to pry technology from American companies—sometimes coercively so, say businesses and the U.S. government.