Two weeks ago, when Homeland Security Secretary Tom Ridge (search) warned of possible Al Qaeda attacks, the "where" was very specific: financial institutions in New York City, Washington and Newark, N.J. The "when," however, was a mystery.

And since Ridge's announcement, the Bush administration has discovered no evidence of imminent plans by terrorists to attack U.S. buildings, a White House official acknowledged Thursday.

Some documents and computer files seized in Al Qaeda (search) raids included surveillance reports of the financial buildings during 2000 and 2001, which prompted warnings Aug. 1 from the White House about possible threats. But nothing in the documents themselves has suggested any attack was planned soon, the official said.

"I have not seen an indication of an imminent operation," the official said, speaking on condition of anonymity with reporters from nearly a dozen news organizations. Investigators are still poring over volumes of the seized information.

Immediately after the warning, police sealed off some streets near the Citigroup Center building and the New York Stock Exchange (search) in New York; put employees at the International Monetary Fund (search) and World Bank buildings in Washington through extra security checks; and added barricades and a heavily armed presence around Prudential Financial Inc.'s headquarters in Newark.

In Washington, Capitol Police blocked all traffic flow near the building and began searching vehicles, even though no new threats to the Capitol had been found. District of Columbia Mayor Anthony Williams protested the measures, calling them "unworkable and unacceptable."

Subway riders in Washington have had to get used to sharing their commutes with police bearing machine guns. New Yorkers have been agitated by FBI warnings of threats posed by helicopters and limousines, while city authorities are beefing up security in advance of the Republican National Convention, which begins Aug. 30.

The White House homeland security adviser, Frances Fragos Townsend (search), told "Fox News Sunday" last weekend that authorities believe discovery of the surveillance has disrupted Al Qaeda's plans to carry out the attacks on the financial buildings.

The FBI and local police still haven't determined whether the surveillance was performed by a single person or several people, and the FBI has not yet identified anyone involved in the surveillance, the White House official said Thursday, adding that the detailed reconnaissance indicated "an awful lot of time and energy put into it."

The surveillance records had been accessed for unknown purposes this spring, months later than authorities had previously disclosed, the official said. The government had said earlier that some files had been reviewed as recently as January.

Another administration official, also speaking on condition of anonymity, said the White House still would have issued the terror alerts even if it had known at the time that the surveillance documents did not point to an imminent operation.

The administration remains deeply concerned about information uncovered separately in the spring suggesting Al Qaeda was plotting a major attack inside the United States — perhaps in August or September — to disrupt the elections, the first official said.

None of the documents or computer files recovered in the recent raids in Pakistan mentioned any election-related plots, the same official said.

This official said unspecified intelligence indicates Al Qaeda's plans for an attack before the election were "more than merely aspirational" but declined to be more specific because it might reveal the information's source. Timing was unclear, the official said, acknowledging that intelligence agencies "wish we had a sense."

Senior U.S. officials — including Townsend, Attorney General John Ashcroft, FBI Director Robert Mueller and national security adviser Condoleezza Rice — have expressed similar concerns since March about possible Al Qaeda efforts to disrupt the U.S. elections.