Key provisions of the Bush-era Patriot Act which allow the National Security Agency's mass surveillance programme have lapsed following a dramatic showdown at the Senate over the weekend.
The Senate held a rare session on Sunday (31 May) to deal with the expiring provisions of the legislation, which came into effect in the aftermath of the 9/11 attacks in 2001. The key provisions of the Patriot Act expired at midnight local time (04:00 GMT).
Meanwhile, Senators are still debating the USA Freedom Act, a replacement bill that would water down bulk data collection. It will not be put to vote at least for the next two days.
Any deal has remained elusive since even those who have lobbied for the NSA are seeking major reforms. The White House said the actions of Senators had led to the "irresponsible lapse".
"On a matter as critical as our national security, individual senators must put aside their partisan motivations and act swiftly. The American people deserve nothing less," it said in a statement.
Bulk collection of data on Americans' communications remains a sticky issue. Though the lapse could be brief, it could still jeopardise the safety of Americans, experts have warned.
Rand Paul, a Republican presidential candidate who played a key role in averting an extension, said: "The point we wanted to make is, we can still catch terrorists using the Constitution. I'm supportive of the part that ends bulk collection by the government. My concern is that we might be exchanging bulk collection by the government [with] bulk collection by the phone companies."
"Tonight begins the process of ending bulk collection. The bill will ultimately pass but we always look for silver linings. I think the bill may be replacing one form of bulk collection with another but the government after this bill passes will no longer collect your phone records."
Paul's tactics in stonewalling a deal have not gone down well with his fellow Republicans who criticised him for his political ambitions.
Republican Senator John McCain, a former presidential contender, said his Kentucky colleague placed "a higher priority on his fundraising and his ambitions than on the security of the nation".