The NSA is shutting down its mass surveillance servers after the intrusive data collection powers exposed by whistle-blower Edward Snowden were allowed to lapse by the Senate.
Objections to the sweeping programmes used to capture US citizens' phone data were led by the Republican presidential hopeful Rand Paul, accused by fellow senators of jeopardising national security for the sake of his personal ambitions.
But the disruption to the key powers of the Patriot Act, brought in after 9/11 in response to heightened terror fears, was hailed by libertarians who say the Government's surveillance powers have become too great.
After a lengthy debate, the Senate voted 77-17 to take up reform legislation known as the USA Freedom Act, which will replace the Patriot Act's bulk phone records programme.
And while the Senate did not act in time to prevent the Patriot Act powers from lapsing, the eventual vote was hailed as at least a partial victory for Barack Obama, who had pushed for reforms to address privacy concerns.
Mass collection of telephone metadata
The Patriot Act had allowed the NSA nearly unobstructed access to information about phone numbers called and the times of the calls. The government claimed they did not have access to the content of those phone calls, but some have raised suspicion over that point.
Now the programme has expired, such records will still be kept by the phone companies but will only be available to the NSA via search warrants.
Roving and lone-wolf wiretaps
Lost to the NSA along with telephone data was the use of two wiretap techniques: roving and lone-wolf. Roving wiretaps allow investigators to track a suspected terrorist known to often switch telephones. Lone-wolf wiretaps – which the Times reports have never been used – allow the tracking of suspects not linked with a specific terror organisation.
Why the delay?
Mr Paul, whose libertarian stance has gained new support for his Republican presidential claim but alienated some traditional backers, claimed the Patriot Act wasted resources that would be better spent targeting those who planned attacks.
The Republican Majority Leader Mitch McConnell disagreed, but was forced to concede that Mr Paul's objections meant the Patriot Act would fail to win majority approval.
Senator John McCain was among Patriot Act supporters who accused Mr Paul of raising money for his presidential campaign. "He obviously has a higher priority for his fundraising and political ambitions than for the security of the nation," he said.
Will it harm security?
The Freedom Act would end spy agencies' bulk collection of domestic telephone "metadata" and replace it with a more targeted system, where the NSA would have to get court approval to access specific data.
A review panel President Obama established in 2013 concluded that the metadata collection programme had not been essential to preventing any terrorist attack.
As for the delay between the two acts, intelligence experts said a lapse of only a few days would have little immediate effect. The government is allowed to continue collecting information related to any foreign intelligence investigations that began before the deadline.Reuse content