Americans are consistently told that the next terrorist attack on U.S. soil is a question of when, not if. The index’s results overwhelmingly agree that the next attack is just a matter of time.

Eighty-four percent of the experts said they believe a terrorist attack on the scale of Sept. 11, 2001, is likely or certain to happen in the next five years. More than a quarter said a 9/11-scale attack is certain to occur in America within the next decade. Asked about the likelihood of a smaller strike akin to the July 2005 London bombings, 91 percent agreed that such an attack is likely or certain by 2016; more than half said that such an attack could happen this year.

But how will the terrorists strike? Roughly two thirds of the experts said that some part of America’s infrastructure—a port, train station, or major landmark—will be targeted. That is no surprise, given that terrorists have repeatedly struck these locales in the past. But it may be more alarming that almost the same percentage predict that the next attack will come in the form of a suicide bombing. These results, says index participant Bruce Hoffman, a terrorism expert at the Rand Corp., “reflect a recognition of how easy terrorism has become.” Such attacks, he says, “are cheap, unpredictable, and difficult to prevent. All that is required is the will to kill and the will to die, neither of which seems in short supply today.”

Americans have never feared a suicide bombing the way the people of Amman or Jerusalem have. But there may be reason to think that will soon change. A recent study by Rand found that 81 percent of all suicide attacks in the past 30 or so years have occurred since Sept. 11, 2001, and the primary motivation for each of these attacks was a military intervention or occupation such as the ongoing operations in Iraq. The odds that America can continue to elude the world’s most popular form of terrorism may be fading fast.