US surveillance powers - which allow the National Security Agency to bulk collect phone data - have expired after the Senate failed to agree a fresh deal.
Key parts of the law, known as the Patriot Act, passed in the wake of the 9/11 attacks, expired at midnight local time after Senators were unable to agree on legislation to extend the powers.
Security services have now lost the power - albeit temporarily - to bulk collect phone records, affecting officers' ability to carry out "roving wiretaps" of terror suspects and to target "lone wolf" suspects.
Republican Senator and presidential hopeful Rand Paul - a prominent critic of surveillance programmes - was the key factor in the failure to extend the programme.
A vote on a revised bill, the Freedom Act, will be held in the coming days where surveillance powers will most likely to be restored.