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China said US chipmaker Micron Technology’s products posed “serious network security risks” as it banned operators of key infrastructure from buying them, in its first big measure against an American semiconductor group.

The Cyberspace Administration of China on Sunday announced that the company, which is the biggest US maker of memory chips, “posed significant security risks to China’s critical information infrastructure supply chain.” As a result, it ordered “critical national infrastructure operators” to stop purchasing products from Idaho-based Micron.

The move follows a seven-week investigation into Micron by the CAC, a probe that was widely seen as retaliation for US efforts to curb China’s access to critical technology. Last October, Washington introduced expansive chip export controls, and the Netherlands and Japan have since followed.