A bipartisan group of 26 US senators has written to intelligence chiefs to complain that the government is relying on a "secret body of law" to collect massive amounts of data on US citizens.
The senators accuse officials of making misleading statements and they are demanding that the director of national intelligence, James Clapper, answer a series of specific questions on the scale of domestic surveillance as well as the legal justification for it.
It's the strongest attack yet by Congress since the disclosures began, and comes after Clapper admitted he had given "the least untruthful answer possible" when pushed on the surveillance issue by senators at a hearing before the latest revelations by The Guardian and The Washington Post.
In their strongly worded letter to Clapper, the senators said they believed the government may be misinterpreting existing legislation to justify the sweeping collection of telephone and internet data revealed by The Guardian.
"We are concerned that by depending on secret interpretations of the Patriot Act that differed from an intuitive reading of the statute, this programme essentially relied for years on a secret body of law," they said.
In a press statement, the group of senators said: "The recent public disclosures of secret government surveillance programmes have exposed how secret interpretations of the USA Patriot Act have allowed for the bulk collection of massive amounts of data on the communications of ordinary Americans with no connection to wrongdoing."
"Reliance on secret law to conduct domestic surveillance activities raises serious civil liberty concerns and all but removes the public from an informed national security and civil liberty debate," they added.
The letter was organised by Oregon Democrat Ron Wyden, a member of the intelligence committee, but includes four Republican senators: Mark Kirk, Mike Lee, Lisa Murkowski and Dean Heller.
They asked Clapper publicly to provide information about the duration and scope of the programme and examples of its effectiveness in providing unique intelligence, if examples exist.
The senators also expressed their concern that the programme itself has a significant impact on the privacy of law-abiding Americans and that the Patriot Act could be used for the bulk collection of records beyond phone metadata.
In addition to raising concerns about the law's scope, the senators noted that keeping the official interpretation of the law secret and the instances of misleading public statements from executive branch officials prevented the American people from having an informed public debate about national security and domestic surveillance.