Google built a prototype of a censored search engine for China that links users’ searches to their personal phone numbers, thus making it easier for the Chinese government to monitor people’s queries, The Intercept can reveal.

The search engine, codenamed Dragonfly, was designed for Android devices, and would remove content deemed sensitive by China’s ruling Communist Party regime, such as information about political dissidents, free speech, democracy, human rights, and peaceful protest.

Previously undisclosed details about the plan, obtained by The Intercept on Friday, show that Google compiled a censorship blacklist that included terms such as “human rights,” “student protest,” and “Nobel Prize” in Mandarin.

Leading human rights groups have criticized Dragonfly, saying that it could result in the company “directly contributing to, or [becoming] complicit in, human rights violations.” A central concern expressed by the groups is that, beyond the censorship, user data stored by Google on the Chinese mainland could be accessible to Chinese authorities, who routinely target political activists and journalists.

Sources familiar with the project said that prototypes of the search engine linked the search app on a user’s Android smartphone with their phone number. This means individual people’s searches could be easily tracked – and any user seeking out information banned by the government could potentially be at risk of interrogation or detention if security agencies were to obtain the search records from Google.

The search engine would be operated as part of a “joint venture” partnership with a company based in mainland China, according to sources familiar with the project. People working for the joint venture would have the capability to update the search term blacklists, the sources said, raising new questions about whether Google executives in the U.S. would be able to maintain effective control and oversight over the censorship.

Sources familiar with Dragonfly said that the search platform also appeared to have been tailored to replace weather and air pollution data with information provided directly by an unnamed source in Beijing. The Chinese government has a record of manipulating details about pollution in the country’s cities. One Google source said that the company had built a system, integrated as part of Dragonfly, that was “essentially hardcoded to force their [Chinese-provided] data.” The source raised concerns that the Dragonfly search system would be providing false pollution data that downplayed the amount of toxins in the air.

Google has so far declined to publicly address concerns about the Chinese censorship plans and did not respond to a request for comment on this story. It has also refused to engage with human rights groups regarding Dragonfly, ignored reporters’ questions, and rebuffed U.S. senators.

In the meantime, pressure on Google has only intensified. On Thursday, 16 U.S. lawmakers wrote to Google CEO Sundar Pichai expressing “serious concerns” about Dragonfly and demanding information about the company’s China plans. Meanwhile, Jack Poulson, a former Google senior research scientist, told The Intercept that he was one of about five employees to have resigned from the company due to the Dragonfly plans.

“I view our intent to capitulate to censorship and surveillance demands in exchange for access to the Chinese market as a forfeiture of our values and governmental negotiating position across the globe,” Poulson told Google bosses in his resignation letter.