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"Indeed, the unfocused dragnet undertaken by Defendants is exactly the type of unrestrained surveillance Congress, including [Sensenbrenner], tried to prevent," the Electronic Frontier Foundation, a civil liberties group, wrote in the brief on behalf of the congressman.

After leaks by Edward Snowden, the NSA acknowledged that it collects records on virtually all U.S. phone calls. The data include phone numbers, call times and call durations, but not the contents of the conversations. The agency says it only "queries" the database a limited number of times for specific national security reasons.

Sensenbrenner's lawyers acknowledge that "relevance" can be a broad legal standard, but insist that Congress intended the provision to limit the NSA's power and prevent the bulk collection of records.

"The vast majority of the records collected will have no relation to the investigation of terrorism at all," the lawyers write, calling the NSA's expansive definition of relevance "unprecedented and dangerous."

The NSA has argued that Congress implicitly endorsed the agency's interpretation of the Patriot Act by reauthorizing the law after being informed of how it was being used. But Sensenbrenner's attorneys argue that only a few lawmakers understood what the agency was actually doing before the program became public.

The NSA allowed lawmakers to view a five-page classified report in a secure location for brief periods in 2009 and 2011, but in the brief for Sensenbrenner, the Electronic Frontier Foundation argues the NSA paper was "sorely lacking in detail" and only had one sentence "that hinted at the breadth of the program."

Even if lawmakers could have learned of the extent of the program, the fact that few actually understood the NSA's activities when they voted to reauthorize the Patriot Act means that they did not endorse the program, the group writes.