The Intercept first reported on Google’s Chinese search project last week. According to the report, the tool takes the form of an Android app, and has already been demonstrated to the Chinese government. The project, reportedly codenamed “Dragonfly,” would be a profound reversal for the company, which in 2010 refused to comply with Chinese government censorship requirements on ethical grounds, and essentially abandoned the market.

Google has not officially confirmed or denied the project’s existence, though The Intercept‘s report was based on internal documents, and multiple reports suggest employees have also seen the app firsthand. The senators’ letter seeks clarity about the project’s existence, and details about its operation. The letter also functions as a laundry list of reasons Google’s possible involvement in such an effort is so disturbing.

Most pointedly, the letter focuses on Google’s recent technology-sharing agreement with Chinese tech giant Tencent, its first such agreement in China. The senators ask whether the agreement was “connected in any way with [Google’s] efforts to enter the Chinese market via the custom search app”. Google is clearly broadening its push for a presence in China—this weekend, reports claimed Google is considering offering cloud services such as Docs and Drive through Chinese partners.

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But the senators’ letter emphasizes widespread claims that Tencent and other Chinese tech giants are closely tied to the Chinese government. Those relationships raise the possibility that Google technology could either be stolen outright or, more subtly, used to create products that obscure Google’s involvement to insulate the company from reputational damage in the U.S.

The letter also asks “which ‘blacklist’ of censored searches and websites” Google would use in a Chinese search product. Chinese internet censorship is active and far-reaching, both blocking thousands of specific websites, and certain discussion topics on social media. Broadly, the goal of the “Great Firewall” is to suppress dissent against the Chinese Communist Party, with prohibited topics including information about domestic scandals, as well as pro-democracy movements and criticism of authoritarian regimes in general.

The letter also rhetorically suggests Google could become entangled in more proactive Chinese “surveillance and social control efforts,” asking whether “Google employees . . . [will] be required to attend the official mandatory trainings on ‘Marxist news values’” required of other digital publishers in the country, and whether Google will “provide information about the search histories of individual users to Chinese government authorities.” Chinese pro-democracy and human rights activists have been repeatedly jailed—sometimes for years—in connection with online activities.

Most broadly, the senators ask: “What has changed since 2010 to make Google comfortable cooperating with the rigorous censorship regime in China?” One clear change is at the top of the company, with Bloomberg last week claiming that Pichai, who became CEO in 2015, is less morally opposed to working within Chinese limits than Google founders Sergey Brin and Larry Page were. As the letter details, China’s internet censorship regime has changed very little, and remains profoundly at odds with the principles of transparency and openness that Brin and Page enshrined. Google’s mission statement, for instance, still claims the company aims to “Organize the world’s information and make it universally accessible and useful.”

The letter was signed by Rubio (R-Fla.), Mark Warner (D-Va.), Tom Cotton (R-Ark.), Ron Wyden (D-Ore.), Cory Gardner (R-Colo.), and Robert Menendez (D-N.J.).