Many Chinese net users were pleased by Google's decision to end censorship
A top US Senator has asked 30 leading internet firms to provide details of their operations in China.
It is ahead of a hearing on how well a voluntary code of conduct, signed by many of the firms, is working.
Senate Democratic Whip Dick Durbin sent letters to firms such as Facebook, Apple, Amazon and eBay asking about their plans to protect human rights.
The letter comes nearly three weeks after Google said it would stop censoring search results in China.
The search giant threatened to pull out of the country altogether after uncovering a hacking attack that it said came from China.
As well as looking at Google's actions in China, the proposed hearing will also examine how well the Global Network Initiative is working.
The voluntary code of conduct was set up following criticism that companies were assisting governments in countries such as China.
Microsoft, Google and Yahoo were among firms to sign up for the deal in 2008.
The initiative stated that "privacy is a human right and guarantor of human dignity" and committed the companies to try to resist demands for restrictions on freedom of speech.
Both AT&T and Facebook told AP that they would respond to the letter by the February 19 deadline while Apple, Amazon, eBay and telecoms firm Verizon did not immediately respond to requests for comment.