John Brennan, director of the government's leading terror-threat analysis unit, the Terrorist Threat Integration Center, told reporters Saturday that bin Laden was likely attempting to "demonstrate that al-Qaeda, as an organization, is still effective, even though they have not, in fact, been able to do something here in the states."
He said the admitted mastermind of the Sept. 11, 2001, attacks was seeking to justify the continued existence of al Qaeda.
The Bush administration left the terror threat level unchanged, despite warning state and local officials that bin Laden's reemergence could portend a new terrorist attack.
"We don't have to go to (code level) orange to take action in response either to these tapes or just general action to improve security around the country," Homeland Security Secretary Tom Ridge said.
He urged Americans to go to the polls Election Day without undue concern. His words and appearance both seemed designed to convey a lack of alarm.
Ridge's department and the FBI issued a memo late Friday to local and state officials, hours after a new videotape of bin Laden surfaced.
"We remain concerned about al Qaeda's interest in attacking the American homeland, and we cannot discount the possibility that the video may be intended to promote violence or serve as a signal for an attack," it said.
Most of the United States has been at code yellow, the midpoint of a five-point color-coded warning scale, for much of the year.
Since August, the terror alert for Washington and Newark, N.J., has stood at orange, or high. At that time, administration officials disclosed al Qaeda had conducted surveillance of four buildings in those cities, as well as in New York City, which is always on orange alert.
Injecting himself into the election, bin Laden said the United States must stop threatening the security of Muslims if it wants to avoid "another Manhattan." While he did not directly warn of new attacks, the al Qaeda leader and Sept. 11 mastermind warned: "There are still reasons to repeat what happened."
While Ridge sought to convey reassurance, he also said the government would strengthen anti-terrorism measures.
Government officials also were scrutinizing a tape aired Thursday by ABC News in which a shrouded man claiming to be an American member of al Qaeda promised attacks that will make U.S. streets "run red with blood." The speaker identified himself as "Azzam the American."
Brennan said investigators were looking "very closely" to see if the two new terror tapes were linked.
In the latest video, bin Laden acknowledged for the first time directly that he ordered the Sept. 11 attacks and said he did so because of injustices against the Lebanese and Palestinians by Israel and the United States.
"Your security is not in the hands of Kerry, Bush or al Qaeda. Your security is in your own hands," bin Laden said, referring to the president and his Democratic opponent, John Kerry. "Any state that does not mess with our security has naturally guaranteed its own security."
The television network Al-Jazeera received the 18-minute videotape at its offices in the Pakistani capital, Islamabad.
Since May U.S. officials had been warning of an al Qaeda attack on one of several major events over the summer — the Olympics and the two national political conventions — or timed to influence the election.
The March 11 bombing of commuter train stations in Madrid, in which nearly 200 died, was seen as dooming the incumbent government of Jose Maria Aznar and elevating his anti-war challenger.
In a campaign dominated by national security, the bin Laden tape came as a late surprise. It was unclear what effect it might have. A CBS News poll conducted from Thursday to Friday saw no significant change in opinions in responses on Saturday, the day after the bin Laden tape emerged.
On the campaign trail, the candidates addressed the tape within familiar language about the war on terrorism: Mr. Bush indicated that the threat was too serious to trust to Kerry, while the Democrat argued that the president had failed to capture bin Laden.
On Sunday, Mr. Bush promised, "we'll eventually get Osama bin Laden. In the meantime, we're destroying his network, slowly but surely, systematically destroying it."