The National Security Agency lost its authority at midnight to collect Americans' phone records in bulk, after GOP Sen. Rand Paul stood in the way of extending the fiercely contested program in an extraordinary Sunday Senate session.

Although the lapse in the programs may be brief, intelligence officials warned that it could jeopardize Americans' safety and amount to a win for extremists. But civil liberties groups applauded as Paul, who is running for president, forced the expiration of the once-secret program made public by NSA contractor Edward Snowden, which critics say is an unconstitutional intrusion into Americans' privacy.