Hundreds of Google employees, upset at the company’s decision to secretly build a censored version of its search engine for China, have signed a letter demanding more transparency to understand the ethical consequences of their work.

In the letter, which was obtained by The New York Times, employees wrote that the project and Google’s apparent willingness to abide by China’s censorship requirements “raise urgent moral and ethical issues.” They added, “Currently we do not have the information required to make ethically-informed decisions about our work, our projects, and our employment.”

The letter is circulating on Google’s internal communication systems and is signed by about 1,400 employees, according to three people familiar with the document, who were not authorized to speak publicly.

The internal activism presents another obstacle for Google’s potential return to China eight years after the company publicly withdrew from the country in protest of censorship and government hacking. China has the world’s largest internet audience but has frustrated American tech giants with content restrictions or outright blockages of services including Facebook and Instagram.