"It's possible there have been challenges, but if so they are still secret," said Alex Abdo, a lawyer with the American Civil Liberties Union, which unsuccessfully tried to overturn the 2008 law as unconstitutional.
Although the Justice Department is required to report to Congress each year on the number of applications it makes to the Foreign Intelligence Surveillance Court, a department spokesman said on Friday he was not aware of any requirement to disclose the number of challenges that companies brought to the court.
The disclosure this week of the NSA's secret and vast phone and email surveillance programs - involving major U.S. telecom and Internet companies - has prompted top Silicon Valley executives to demand greater transparency.
"We understand that the U.S. and other governments need to take action to protect their citizens' safety - including sometimes by using surveillance," Google Chief Executive Officer Larry Page and Chief Legal Officer David Drummond said in a joint statement. "But the level of secrecy around the current legal procedures undermines the freedoms we all cherish."
The technology companies, including Apple Inc, Yahoo Inc, Microsoft's Skype, AOL and PalTalk, said they had not heard of Prism before. Former intelligence analysts said that was likely because the NSA only used that name internally.
The Washington Post first reported that Prism had voluntary cooperation from the companies but later wrote that they had been directed to comply with requests for help from the Attorney General. The Post initially reported that the companies gave officials access to their servers, then later cited a classified memo stating that analysts instead could issue queries to equipment installed at the companies.
On Saturday, the New York Times reported that the equipment had been strenuously negotiated with the companies and was the computer equivalent of a locked room for sharing data.
"Historically, you hear about such 'partnerships,' and intelligence has been doing things like this for a very long time," said former NSA analyst Ron Gula, now chief executive of Tenable Network Security. "What's changed is the volume."
The extent of that change in volume remains unknown. Though executives at the technology companies vigorously denied handling bulk requests, mechanical queries by agencies could still produce large amounts of data.
Google and Twitter, which is notably absent from the NSA slides about Prism that were published by the Washington Post, have gone to regular courts to oppose some other requests for data on their users.
These requests include National Security Letters, which are issued by the FBI and do not need to be approved by a court. Though more than 90 percent of those letters have come with a prohibition on their disclosure, a federal judge in San Francisco ruled recently those gag orders are unconstitutional.
Sign up for CIO Asia eNewsletters.