US congressmen have condemned major IT firms including Microsoft and Google for helping China censor the internet.
Google's decision has been hotly debated
Members of the Congressional Human Rights Caucus said four US firms were putting profits before American principles of free speech.
The hearing follows Google's decision to block politically sensitive terms from its new Chinese search site.
Microsoft, Yahoo, Cisco and Google were also criticised for not attending the hearing in Washington.
Yahoo and Microsoft defended themselves in a statement, saying they did not have the power to force change on governments.
Their services had, they said, "enabled far wider access to independent sources of information for hundreds of millions of individuals in China and elsewhere".
Tom Lantos, top Democrat on the House International Relations Committee, said: "There has been a string of disturbing incidents in which US-based Internet companies have bowed to pressure from Beijing."
Chinese computer users are barred from some websites
"These massively successful high-tech companies, which couldn't bring themselves to send representatives to this meeting today, should be ashamed. They caved in to Beijing for the sake of profits."
The Chinese government enforces strict laws on internet use, blocking content it considers a threat, including references to the Tiananmen Square massacre and notable dissidents.
Google.cn, launched last month, complies with these guidelines.
The group Reporters Without Borders alleges that Yahoo provided information to the Chinese authorities that helped them identify and convict Shi Tao, a journalist who criticised human rights abuses in China.
Microsoft pulled an internet posting by a Chinese government critic, after being ordered to do so.
Information 'will out'
Carolyn Bartholomew, an internet expert, told the hearing that China was becoming the biggest internet hub in its region, and was exporting filtration technology that allowed other "oppressive regimes, including North Korea and Uzbekistan" to control and use the web for their own ends.
Congress will hold a more formal inquiry in two weeks' time.
The firms have said they will attend that process.
In Portugal on Wednesday, Microsoft chairman Bill Gates said attempts to prevent the spread of information would ultimately fail.
"The ability to really withhold information no longer exists... if there is a desire by the population to know something, it is going to get out," he said.