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In the wake of the US Navy Seals鈥� killing of Al Qaeda leader Osama bin Laden on Sunday, security in American cities has been heightened amid concern that Al Qaeda or its sympathizers may try to retaliate against the United States.

Travelers will see more intense screening at airports. In subways and other areas with heavy foot traffic, police, often with K-9 units, are more apparent. Plainclothes officers are keeping an eye on hotels where dignitaries often stay. An elite unit of Marines trained to handle chemical and biological weapon attacks has been recalled from Japan. Many other security measures that are not readily apparent are doubtless being taken.

How long will this extra vigilance continue? And how will the antiterrorism forces know it鈥檚 safe to relax?

The Department of Homeland Security has not raised the threat level since Bin Laden's death, saying it takes that step only in the face of specific or credible information. But some terrorism experts in the US say it鈥檚 not a matter of 鈥渋f鈥� there will be a revenge attack but of 鈥渨hen鈥� 鈥� and their level of concern is high.

鈥淩evenge is very important to them,鈥� Tawfik Hamid, chairman for the Study of Islamic Radicalism at the Potomac Institute, a think tank in Washington, says of jihadists. 鈥淚t is part of the mental process, part of the culture.鈥�

For Al Qaeda or groups allied with it, any attack would be designed to inflict 鈥渢he maximum level of pain,鈥� says Mr. Hamid. Attacks on groups of children or tourists, or a bar that is popular with American students in a country abroad, are among the possibilities because they are soft targets that are hard to defend, he says. 鈥淭hey will look for something that is the easiest and fastest way to tell the US this is revenge for Bin Laden.鈥�

Hamid expects such an attack to occur within two or three months. If an attack is delayed for years, he says, it becomes harder to characterize it as revenge.

If Bin Laden's demise prompts jihadists to accelerate plans for an attack, that might actually help the US intelligence community, he says. 鈥淚f they become emotional and feel they have to move fast in their thirst for revenge, they can make mistakes, and we can infiltrate them and cause more destruction to them.鈥�

Hamid, author of "Inside Jihad," is one who would know how a radical terrorist would think. He is a former member of the radical Islamist organization Jamaa Islamiya with Ayman al-Zawahiri, who later became second in command at Al Qaeda. After 9/11, he became more active in explaining terrorists鈥� mind-set.

Any terrorists who have plans to strike will now fast-track them, agrees terrorism expert Frank Cilluffo, director of George Washington University鈥檚 Homeland Security Policy Institute.

Still, if there's no attack within several months, he warns, 鈥渇rom a security standpoint, you can鈥檛 go to sleep.鈥�

鈥淵es, there is an immediate window,鈥� he says, 鈥淏ut you cannot assume, if there is a window, they intend to strike [at that moment].鈥� In the past, he notes, Al Qaeda has looked for large-scale types of attacks.

That鈥檚 one reason it is a good idea to bring back the Marine unit from Japan, experts say.

鈥淚t is sensible to be vigilant in all areas,鈥� says Gary LaFree, a professor at the University of Maryland and director of the National Consortium for the Study of Terrorism and Responses to Terrorism (START). 鈥淚t would be crazy to rule out a ... chemical or biological attack.鈥�

According to a START briefing paper, of the 25 attacks since 1998 that killed 25 people or more, Al Qaeda was involved with 16.

In fact, Al Qaeda now has connections to 30 or 40 terror groups, says Mr. LaFree. 鈥淚t鈥檚 more like a franchise. And they are not going away,鈥� he says.

Security organizations need to make it harder for terrorist organizations to plan by conducting some of their activities at random, says Mr. Cilluffo. 鈥淚t has to be done right, where you don鈥檛 let the adversary game the system,鈥� he says.

For Hamid, the solution lies in dissuading radicalized Muslims from carrying out terrorist acts by understanding what matters most to them. 鈥淔or example, if you say, 'I will kill you and all other Muslims,' the terrorist will say, 鈥業 don鈥檛 care.鈥� " However, if the US and its allies can show how a terrorist act may make a Muslim woman destitute and perhaps force her into prostitution to live, it will have a greater deterrent effect, Hamid says.

鈥淔or a Muslim, a Muslim woman鈥檚 honor is vital,鈥� he explains. 鈥淚t will make [potential terrorists] think twice.鈥�