In May, the FCC rejected China Mobile’s application to become a telecom provider in the US, citing “substantial and serious” law enforcement and security risks. In their letter, the Senators reportedly warn that China Telecom and China Unicom could pose similar threats and could reroute communications traveling on their networks through China. When the FCC denied China Mobile’s application, it said it was reviewing existing approvals. In a statement provided to Reuters, FCC spokesperson said Chairman Ajit Pai “has made it clear that the Commission is reviewing other Chinese communications companies such as China Telecom and China Unicom.”

While the FCC may already be investigating the two telecoms, the letter shows that Congress is willing to apply pressure to Chinese technology firms, even as the Trump administration lifts some restrictions on companies like Huawei.