The US National Security Agency was blocked by a judge from carrying out plans this week to begin destroying phone records collected for surveillance after a privacy group argued they are relevant to lawsuits claiming the practice is unconstitutional.
US district judge Jeffrey White in San Francisco ordered the agency on Monday to retain the records, and scheduled a hearing for March 19 on whether they can be destroyed.
The NSA had planned to dispose of the records following a March 7 ruling by the US Foreign Intelligence Surveillance (Fisa) Court in Washington.
The Electronic Frontier Foundation (EFF), an internet privacy group based in San Francisco, asked White for a temporary restraining order, saying the records may be used as evidence in its lawsuits challenging NSA spying and are covered under preservation orders in those cases.
The NSA was prohibited from destroying "any telephone metadata or 'call detail' records", White said on Monday.
The surveillance court, in its ruling, barred the NSA from keeping the records for more than five years because the privacy rights of the people whose phone data was swept up by the agency trump the need for the information in litigation.
The retention of phone records has emerged as one of the most contentious issues for the government stemming from the documents that were leaked by former security contractor Edward Snowden.
The records collected under an NSA program consist of "metadata" including the numbers used to make and receive calls and their duration. They do not include information about the content of the communications, or the names, addresses or financial information of parties, according to government filings.
The EFF challenged the NSA telephone surveillance in a complaint filed in July alleging that it illegally erodes the free-speech rights of religious, environmental and human-rights groups. The records were also relevant to a pending 2008 case challenging NSA surveillance, and the NSA was on notice in both cases that it should preserve evidence, said Cindy Cohn, legal director at EFF.
Last week's ruling by the Fisa court was in response to a government bid to waive the requirement records be destroyed after five years because they might prove relevant to such lawsuits.
The US request to hold the records indefinitely "would further infringe on the privacy interest of the United States persons whose telephone records were acquired in vast numbers and retained by the government for five years to aid in national security investigations," wrote US district judge Reggie Walton, the presiding judge of the surveillance court.
The March 7 Fisa Court ruling was "based on a mistaken belief that no preservation order existed for the material", Cohn said.
"If the government proceeds with its planned destruction of evidence, the evidence will be gone," Cohn said. "This is by definition irreparable."