The US has lowered its terror alert level from orange to yellow, Homeland Security Secretary Tom Ridge says.
The US has stepped up security in the air
The danger of attacks during the festive season had passed, Mr Ridge said, but some locations and economic sectors would maintain higher security.
They are said to include major states such as New York and Washington and industries such as civilian airlines.
The alert was raised to orange on 21 December, after intelligence allegedly pointed to impending attacks.
There is a five-colour system of threat alert, with red the highest. Friday's lowering of the alert means the threat level has changed from "high" to "elevated".
Mr Ridge expressed relief that no attacks had occurred, saying specific plans had been thwarted.
"I know we are all thankful nothing happened" during the three weeks of the orange alert, Mr Ridge told a news conference on Friday.
He said he had decided to lower the threat level after a "careful review of the available intelligence" - but warned the nation not to let its guard down.
US STATES OF ALERT
Mr Ridge said extra vigilance would be maintained in "certain locales and sites in private sectors" - including commercial airlines.
And some cities, including New York, Washington and Las Vegas, have been quietly told to remain vigilant.
New York has been permanently on high alert since 11 September 2001, and any decision to lower its status would have to be taken by the New York Police Department.
The BBC's Damian Fowler in New York says there is still a very visible police and National Guard presence in key public buildings, and New Yorkers are getting used to the idea of being in a heightened state of alert.
And while Mr Ridge praised international co-operation, he said more had to be done to establish global security standards.
At the time the threat level was raised, US officials pointed to "credible reports that al-Qaeda was hoping to mount attacks equal to or greater than those of 11 September 2001".
RECENT US SECURITY STEPS
21 Dec: US increases alert level to orange
24 Dec: France cancels six flights to Los Angeles
30 Dec: US requires certain foreign airlines to carry armed guards
31 Dec: Security is tight on New Year's eve, with military patrols at Times Square and sharpshooters in Las Vegas
2-6 Jan: Flight BA 223 from London to Washington DC delayed
5 Jan: New rules require visa holders entering US to be fingerprinted and photographed
7 Jan: Secret radiation tests have been carried out in major US cities amid fears of a "dirty bomb", say reports
The US took a number of steps to tighten security after declaring an orange level of alert, including asking certain international airlines operating routes to the US to carry armed sky marshals on board.
A number of international flights to the US were also cancelled.
Tim Brown, a senior fellow at the GlobalSecurity think tank, told the BBC that the US would continue to be put on orange alert in holiday periods for the foreseeable future.
At the same time, the country is routinely put on high alert during holiday periods, because large gatherings are likely to prove potential terrorist targets.