Ever since the Sept. 11 attacks, airports have probably been the most heavily guarded sites when it comes to preventing terrorist attacks.
And yet the most recent terrorism plot from Yemen involved an attempt to blow up a U.S. airliner with a bomber wearing a difficult-to-detect explosive bomb in his underwear, according to U.S. officials.
Why do terrorist groups keep trying to defeat the multiple layers of security at airports when there are so many soft targets?
For one, a plane heading into the U.S. represents the first available target to strike against a large number of Americans. It doesn't require reaching the U.S. first, and then acquiring a weapon and launching an attack from U.S. soil.
Also, terrorist groups have learned from previous attacks on planes.
"Terrorists like to do what they know how to do," says terrorism analyst Jessica Stern.
But the difficulty of breaching airport security does appear to be generating other approaches.
Two Different Types of Plots
Stern says she sees two trends. One involves developing new and more sophisticated techniques for evading security measures and attacking airplanes.
The other involves "looking for low-tech ways to attack softer targets," she says. This is a way of encouraging "leaderless resistance," says Stern, the author of Terror in the Name of God.
For example, the latest issue of Inspire, the jihadi magazine produced by the Yemen-based group al-Qaida in the Arabian Peninsula, includes an eight-page feature that encourages readers to start wildfires in Australia and the United States.
It recommends that would-be saboteurs in the United States study weather patterns in order to determine when vegetation will be dry and winds favorable for a wildfire.
It specifically suggests Montana as a good site for practicing pyro-terrorism, because of the residential housing that is in wooded areas.
Stern says the aim of terrorism is to frighten the public and push governments into overreacting — so spectacular, random-seeming attacks like airplane bombings work well.
"Terrorists do really aim for what we call symbolic targets," she says. "Terrorism is a form of theater, so they're going to hit targets that will make us maximally afraid, and inflict the maximum amount of humiliation."
In that sense, she says, arson in populated forest areas could be "a good second best" for a target.
A Range of Vulnerabilities
Security analysts have pointed to dozens of potential terrorist targets and vulnerabilities, from military bases to passenger trains, chemical plants to storage for liquefied natural gas.
Former CIA agent Charles Faddis says he expects that there will be more attacks on targets that, by their nature, are hard to defend.
Faddis, the author of Willful Neglect: The Dangerous Illusion of Homeland Security, says he particularly fears situations where suicide gunmen might attack people at a public event.
"There are an infinite number of targets where you can find large numbers of people — college campuses, pro sports events," he says.
Even where such events have security screening, Faddis adds, they often don't have armed guards, so a determined, suicidal shooter would be hard to stop.
A Focus on Resiliency
That problem is causing analysts to rethink the balance between guarding against an attack and recovering from one.
"We've got to recognize that we're never going to be able to answer the question, 'Are we safe?' with a definitive 'Yes,' " says Juliette Kayyem, a lecturer on public policy at Harvard's Belfer Center. "So how do we prioritize risks?"
Kayyem says the government still needs to keep attention on "high consequence" targets, such as nuclear power plants or toxic chemical storage facilities.
But she says the country also needs to focus on resiliency — the ability to recover from destruction ranging from terrorist attacks to natural disasters.
"When you prepare society to deal with destruction, you reduce the incentive for terrorist attacks," says Steve Flynn, co-director of the Kostas Research Institute for Homeland Security at Northeastern University.
Flynn cites the example of forest fires. "If you can respond capably to someone who sets a fire, there isn't a lot of incentive for someone to set them," he says. "And we should be ready to deal with them anyway, because Mother Nature is the ultimate arsonist."
Flynn has his own list of critical targets that need strong security measures, beginning with refineries and petrochemical plants. "Why import a weapon," he asks, "when we already have them pre-positioned around urban areas?"
Government also needs to take steps to protect the power grid, he says, because if assets such as power substations are destroyed, they can take from one to two years to rebuild.
Limits Of Security
But Flynn warns against overstating what government can do to protect against attacks.
"People have been fed this paternalistic thing about fears," he says. "We need to tell the public, 'Here's the limit of what we can do. Here's what you need to live with.' "
Juliette Kayyem says experience shows that the American people are up to it. "Studies show that when bad things happen, people don't panic, they don't run for the hills. They help the people around them."