Television and press pictures showed heavily armed police officers protecting the buildings in question, with civilians working in the area subject to heightened checks.
However, officials in Washington later disclosed that much of the information about Al-Qaeda's possible intentions was not new but dates back some years. True, the material had only just come into the hands of U.S. and Pakistani intelligence officers, having been captured in a raid on Al-Qaeda suspects in Pakistan in late July.
But the age of the material, in some cases four years old, led to questions about whether the dramatic raising of the alert level to "orange" -- one step below the maximum alert -- was necessary.
Dublin-based security analyst Shaun Boyne said the decision must be seen in the context of the situation created by the unprecedented terror attacks on 11 September 2001.
"America was so shocked by 9/11 that unusual measures are now being taken to deal with any sort of terrorist threat," Boyne said.
But Washington is a 100 percent political town, and members of the Democratic Party immediately sought to politicize the raising of the alert. They linked its timing to the Democratic National Convention on 26-29 July, which chose John Kerry as the party's challenger to Republican President George W. Bush.
In other words, raising the alarm would mean that media attention would swing away from the Democrats to the alleged Al-Qaeda threat.
White House security adviser Fran Townsend specifically denied that the alert had anything to do with the Democratic Convention or was related to any domestic political considerations.
Analyst Boyne said U.S. authorities are anxious to avoid appearing to "sit" on information, as they were accused of doing in the case of the 2001 attacks on the World Trade Center and the Pentagon, when the attacks burst without warning upon an astonished country.
"My guess is that [the authorities] do not want to be accused later on of failing to share information with the public," Boyne said. "The security establishment over there [in the United States] was criticized over the way that intelligence was handled prior to 9/11 and I think they are adopting a policy now of being as open as possible about these threats; and the fact that the intelligence is old does not mean that there is not a threat still there."
Homeland Security chief Tom Ridge defended the raising of the alert, suggesting that the captured information remains relevant because Al-Qaeda had updated it as recently as January.
"Al-Qaeda often plans well, well in advance. But we also know that they like to update their information before a potential attack. So I don't want anyone to disabuse themselves of the seriousness of this information simply because there were some reports that much of it is dated, that it might be two or three years old."
Cooperation between the United States and Pakistan on fighting terrorism appears to have been increasingly effective lately, with 18 suspected Al-Qaeda operatives rounded up in various raids in Pakistan within the last three weeks.
Pakistani Interior Minister Faisal Saleh Hayat indicated that an important Al-Qaeda figure is among those captured. He did not identify the man but said he has "a multimillion-dollar bounty on his head."
In his comments in Washington, Tom Ridge issued a challenge to terrorists, saying that despite the need for alerts, America's open lifestyle will continue unchecked.
"We know what you [terrorists] want to do, but we are not going to let you do it," Ridge said. "We will not become fortress America. We are going to continue to lead our lives and keep moving forward and hold fast to our freedoms. Nothing -- nothing will ever change that."