The US government's interpretation of its authority under the Patriot Act is so broad that it could justify mass collection of financial, health and even library records of innocent Americans without their knowledge, a civil-liberties lawyer warned at a hearing on a lawsuit challenging a federal phone-tracking programme.
A government lawyer, Stuart Delery, insisted that counterterrorism investigators would not find most personal information useful. Analysis of phone records, however, has become an essential - and legal - tool to "find connections between known and unknown terrorists", he argued.
But Jameel Jaffer, of the American Civil Liberties Union, told a judge in federal court: "If you accept the government's theory here, you are creating a dramatic expansion of the government's investigative power."
US District Court Judge William Pauley reserved decision on an ACLU request to halt the National Security Agency surveillance programmes pending the outcome of its lawsuit against President Barack Obama's administration.
The ACLU sued earlier this year after former NSA analyst Edward Snowden leaked details of the secret programmes that critics say violate privacy rights. The NSA-run programmes pick up millions of telephone and internet records that are routed through US networks each day.
The ACLU has asked the judge to declare the programme unconstitutional, arguing that it exceeds the congressional authority provided by the Patriot Act, which Congress passed after September 11, 2001, and reauthorised in 2005 and 2010.
Obama has defended the programme and said privacy must be balanced with security.