A group of Republican Senators said Huawei presents a “real and urgent” security threat in a letter to Microsoft President Brad Smith after he publicly called for more evidence behind the Trump administration’s reasoning for cutting off the Chinese telecommunications giant from U.S. technology.

As first reported by Bloomberg, the letter from U.S. Senators Tom Cotton, Marco Rubio, Rick Scott, Mike Braun and Josh Hawley lists eight public examples of espionage and technology, as evidence of the threat Huawei proposes. One of those cases was Huawei’s alleged theft of designs and parts of T-Mobile’s top secret cell phone testing robot, nicknamed “Tappy.” T-Mobile won a lawsuit against Huawei, but it was only awarded $4.8 million in damages, a far cry from the $500 million it sought.

“We appreciate Microsoft’s communications with our offices and your understanding of the threats posed by Huawei,” according to the letter. “We also understand that many American companies have conducted business in good faith with Huawei and other Chinese telecommunications companies. While the U.S. government and American industry must take certain steps to protect our people and our telecommunications infrastructure, we do not want to cause undue harm to those American companies.”

“We believe, however, that a review of publicly available evidence indicates that the security concerns about Huawei are real and urgent.”

Smith told Bloomberg Businessweek last month that the company has asked regulators to explain why the administration decided to put Huawei on an “entity list” earlier this year. The move bars companies, including Microsoft, from selling technology to Huawei without government approval.

“Oftentimes, what we get in response is, ‘Well, if you knew what we knew, you would agree with us,’” Smith told Bloomberg. “And our answer is, ‘Great, show us what you know so we can decide for ourselves. That’s the way this country works.’”

The Trump administration has gone back and forth on Huawei, which has become a central part of trade wars between the U.S. and China. Though the administration put Huawei on an export blacklist earlier this year, it later offered a grace period that was extended until November.

In the interview with Bloomberg last month, Smith appealed to Trump’s real estate background in his critique of the administration’s handling of Huawei.

“To tell a tech company that it can sell products, but not buy an operating system or chips, is like telling a hotel company that it can open its doors, but not put beds in its hotel rooms or food in its restaurant. Either way, you put the survival of that company at risk.”