American police patrols have increased in major US cities, but on a five-point scale for terrorist threats the military remained hovering around three – ‘Level Bravo’.
Washington warned travellers of a possible bin Laden backlash.
Yet a former CIA chief is not too pessimistic about anti-American violence.
Charlie Allen, former Assistant Director of the Central Intelligence Agency, said: “I think we can expect it but I think we have to be patient. I don’t think we’ll see an attack immediately. It can be months, it can be years, before the attack occurs. But there will be efforts to avenge the death of Osama bin Laden.”
Homeland Security Secretary Janet Napolitano said: “Our posture includes both seen and unseen measures, and will continue to respond appropriately to protect the American people.”
Officials spoke of “heightened vigilance” for possible retaliation for the killing of the figurehead of al Qaeda terrorism operations.