An hour after President Obama announced that Osama bin Laden was dead, a midnight bulletin flashed across the country to state and local law enforcement officials, warning them that a suddenly leaderless Al Qaeda would probably "retaliate" and "continue to pursue attacks" against the United States.
The caution from the Homeland Security Department in Washington escalated Monday as national security officials, terrorism experts and the White House agreed that future strikes could likely be triggered from a new power struggle inside Al Qaeda or by some lone wolf or "micro-terrorist" plotting in the U.S. to personally even the score for Bin Laden's death.
Around the country, airports beefed up inspections, mass-transit police heightened patrols and cities such as Los Angeles, New York and Chicago increased their security measures. Abroad, U.S. embassies and other foreign facilities were placed on high alert, and U.S. citizens were strongly advised to be careful if traveling or living overseas.
In the midnight bulletin to state and local officials, Washington warned that anything could happen. "Everyone is extremely sensitive to the fact that there will at least be an attempt for a retaliatory attack," said one U.S. intelligence official.
Yet officials strongly underscored that Al Qaeda had a tradition of being patient, and was willing to take its time to make a bigger splash. With that in mind, the consensus was that terrorists would probably strike again; when and where remain unknown.
"The enemy is out there," said Sen. Joe Lieberman (I-Conn.), chairman of the Senate Homeland Security Committee. "My own great concern in the days ahead is a so-called lone wolf."
"Though Bin Laden is dead, Al Qaeda is not," CIA Director Leon E. Panetta told his employees in an early morning message, encouraging them to keep up their guard. "Terrorists almost certainly will attempt to avenge him."
"They are a wounded tiger," said White House counter-terrorism advisor John Brennan, acknowledging that Al Qaeda will try to regroup. "But they still have life left in them."
Experts on terrorism said the list of possible targets was long and the possible perpetrators would be difficult to identify.
Christine Parthemore, a fellow at the Center for a New American Security, a nonpartisan research group in Washington, said terrorists could try to hurt the U.S. economy with some kind of attack on Middle Eastern gas and crude oil. "Top concerns in the coming months should include reprisal attacks within Saudi Arabia, where petroleum infrastructure has always been targeted," she said.
Frank Cilluffo, who was White House domestic security advisor to President George W. Bush, said U.S. officials were concerned that the next attack could be against a "soft" target like a crowded mall or restaurant, and the shooter could be an American who never had to leave the U.S. to link up with a terrorist organization. "Something more quick-moving and fluid, soft targets," Cilluffo said.
It is much easier to detect violence being planned by an organization, but individuals acting alone are nearly impossible to stop, said Rick Nelson of the Center for Strategic and International Studies, a Washington-based think tank.
"Anybody can go on a website and say, 'I want to kill Americans.' How do I know when one of those individuals is actually going take action?"
Near the World Trade Center site in New York, Mayor Michael R. Bloomberg spoke as helicopters whirred overhead and the area flooded with police, journalists and wary tourists.
"Osama bin Laden is dead, and New York City's spirit has never been stronger," Bloomberg said.
But Bloomberg acknowledged the threat hanging over the city. "There is no doubt we remain a top target, and the killing of Bin Laden will not change that," he said.
His predecessor, Rudolph W. Giuliani, also appeared near the site. He praised the administration in Washington and the military for "the courage" to take action against Bin Laden.
But he too warned of "short-term dangers," saying some of the elation on Monday was a bit premature, that the fight goes on.
"I know there are going to be people who are going to want to do damage to us," Giuliani said.