"The secrecy surrounding the government's extraordinary surveillance powers has stymied our system of checks and balances," wrote Laura Murphy, director of the ACLU Washington Legislative Office. "Congress must initiate an investigation to fully uncover the scope of these powers and their constraints, and it must enact reforms that protect Americans' right to privacy and that enable effective public oversight of our government."
Facebook called some of the allegations inaccurate. "We do not provide any government organization with direct access to Facebook servers," the company said in a statement attributed to Chief Security Officer Joe Sullivan. "When Facebook is asked for data or information about specific individuals, we carefully scrutinize any such request for compliance with all applicable laws, and provide information only to the extent required by law."
"Yahoo! takes users' privacy very seriously. We do not provide the government with direct access to our servers, systems, or network," a Yahoo spokeswoman said.
Google also said it reviews all user data requests carefully. "From time to time, people allege that we have created a government 'back door' into our systems, but Google does not have a 'back door' for the government to access private user data," the company said in a statement.
Apple also denied involvement, saying it had never heard of PRISM, according to news reports.
(Grant Gross in Washington and Zach Miners in San Francisco contributed to this report)