NEW YORK CITY—On Monday morning, hours after U.S. forces killed Osama bin Laden, subway platforms and cars here held more than their usual share of cops. Police officers will continue to clock overtime in the coming days and potentially weeks in the Big Apple. It's been almost ten years since al Qaeda sleepers brought down the World Trade Center towers, but the city isn't taking any chances with news of bin Laden's death.

Many security officials, not only here but in Washington, D.C., too, are expecting al Qaeda and sympathetic terrorist groups to seek some kind of retribution. U.S. forces fatally shot Bin Laden on Sunday. The cycle of response and counter-response has been a regular staple of the "war on terror" for at least the past decade, and nobody expects it to end with Bin Laden's death. "Clearly, this is an event that will almost certainly produce at least an attempted response," says Brian Jackson, a senior physical scientist and terrorism analyst at RAND Corporation. "In these kind of back and forth conflicts with non-state groups that is one of the inevitable dynamics."

In recent years, Osama bin Laden and his inner circle have lost much of their ability to coordinate terrorist activity, but "franchise" groups continue to plot and act at the regional level, like Al Qaeda in the Arabian Peninsula, or individuals like Faisal Shahzad, who attempted to bomb Times Square last year. Whatever plans were being hatched before bin Laden's death may be fast-tracked. Those plans could include targets in the U.S. and Europe, government officials in countries such as Pakistan, or even tourists in countries such as Egypt, say security experts. Those most at risk, of course, are those serving abroad in the military.

Most likely, terrorists will continue to rely on the tactics of the past: attacks on aviation and the use of conventional explosives to spread mayhem. But terrorists have shown that they learn from their failures. Recently they have switched from suicide attackers like Richard Reid, who was caught trying to ignite a bomb in his shoe in December, 2001, to planting explosives in cargo. Last year, for instance, British and American officials intercepted bombs packed in printer cartridges and sent through the mail.

"There is always going to be somebody out there to say I want to get even for my hero," says Jospeh King, professor of terrorism and organized crime at John Jay College of Criminal Justice.  "If I were them I'd be planning my revenge now."

The Obama administration has not taken visible steps to bolster security and have downplayed any potential threat in the wake of Bin Laden's death. "Any type of event like this, it is very prudent for us to take measures so that we can ensure that the security measures we need to institute here and throughout the world are in place," says a senior administration official. "This is just something that we normally would do. We don't have any specific threats at this time related to this. But we are ensuring that every possible precaution is taken in advance."

Unlike the years of color-coded alerts immediately following 9-11, the U.S. Department of Homeland Security has declined to signify any change in threat through its new system of alerts. "We remain at a heightened state of vigilance," said Secretary of Homeland Security Janet Napolitano in a statement. "Our security posture, which always includes a number of measures both seen and unseen, will continue to respond appropriately to protect the American people from an evolving threat picture in both the coming days and beyond."

That said, security will only ever be as good as the individual police officer or federal agent tasked with a particular security effort, whether screening cargo containers or observing critical infrastructure. "There are 20 million containers that come into the U.S. every year and less than one-half of one percent are even looked at," says King, who used to be the assistant special agent in charge of New York and chief of the counter-terrorism unit for the U.S. Customs Service.

The purpose of increasing police presence in U.S. cities like New York may be as much to provide reassurance to the public as to intercept terrorists. "It's kind of a feel-good operation unless there's specific intelligence," King notes. "They should be doing it, though, because it gives increased comfort to people and it is hardening a target."

Other security experts argue, however, that such extra efforts are "simply security theater," wrote Bruce Schneier, chief security technology officer at BT, in an email to Scientific American. He added that there is "nothing" to anticipate in the way of a backlash and that existing security measures have been effective. “I'm more worried about the 42,000 people who die each year in car crashes,” he says. “That's more than a 9/11 each and every month."

Ultimately, the best security response is to minimize the opportunities for terrorism, such as shortening ticketing and security lines at airports. "The variable that gives you the most leverage on reducing casualties was how quickly you processed people through ticket lines and security lines," says Jackson, referring to a RAND study on improving security at Los Angeles International Airport. "If you moved people through fast and so the density of people was lower then, even if an attack was successful, you have prevented a significant number of casualties just because people weren't there to be hurt in the first place."