The US administration admits that new warnings of attacks on American cities were based on information gathered by al-Qaeda up to four years ago.
Security has been stepped up in New York and other cities
Security was tightened around US financial institutions earlier this week after raids in Pakistan recovered documents reportedly naming them.
Homeland security adviser Frances Townsend said some of the information recovered was collected in 2000/2001.
But she said some may have been updated "as recently as January of this year."
On Sunday, Homeland Security chief Tom Ridge said the US had received "new and unusually specific information about where al-Qaeda would like to attack".
"In light of new intelligence information, we have made the decision to raise the threat level for this sector in these communities to bring protective resources to an even higher level," Mr Ridge said.
Some have suggested that the timing of the latest US government warning is designed to knock presidential challenger John Kerry off the front pages after his nomination as the Democratic Party's candidate last week.
But Ms Townsend said: "It had nothing to do with the Democratic National Convention."
Her comments followed reports in leading American newspapers that US officials were unsure if Osama Bin Laden's network was still conducting surveillance on the sites named as potential targets.
She confirmed: "You can't tell from the intelligence
itself whether or not those individuals [who amassed it]
are still here."
Employees turned up for work this week despite specific US government warnings naming the New York Stock Exchange, the Citigroup Center building in New York, the International Monetary Fund and World Bank buildings in Washington DC, and Prudential Financial's headquarters in Newark, New Jersey.
The raids in Pakistan reportedly turned up hundreds of photos, sketches and written documents, which included details on the number of pedestrians passing named buildings, and whether explosives would be able to melt the steel holding them up.
A computer and communications expert reportedly linked to al-Qaeda, Mohammad Naeem Noor Khan, and one of America's most wanted terror suspects, Ahmed Khalfan Ghailani, were among those held by the security forces.
'Nation in danger'
BBC diplomatic correspondent Jonathan Marcus says some leading Democrats believe that the Bush administration is playing the terrorism card for all it is worth.
But he adds that whatever lies behind this heightened alert, the broader threat remains real enough and is likely to grow as November's election gets closer.
President George W Bush has described the US as a "nation in danger".
He has asked Congress to clear the way for a new national intelligence director, and announced the creation of a national counter-terrorism centre to collect and analyse data on suspected terrorist activities.
The measures follow recommendations made by the Senate commission that investigated the 11 September attacks.