Al Qaeda has made a horrifying—if bizarre—advance in terrorist tradecraft. As recently reported by my friend Frank Gardner, security correspondent for the BBC, the suicide bomber who tried unsuccessfully to blow up Saudi Arabia's counterterrorism chief in August actually had the explosives inside his body. It's possible the bomb—which was made from materials that wouldn't set off metal detectors—was swallowed or stitched into him in some fashion, but according to one usually authoritative Saudi official, the explosives had been inserted in the terrorist's rectum.

The target, Prince Ahmed bin Nayef, escaped with minor injuries, but the blast left a large crater in the floor and pieces of the bomber all over the place, including an arm blown through the ceiling. "Fortunately, the force of it went in the wrong direction," said the same veteran of the Saudi intelligence services, speaking privately about this explosive suppository, and stifling a laugh. (Article continued below...)

But this is no joke. As Gardner pointed out in his report, if suicide bombers are willing to go to such lengths, there isn't much that conventional airport security can do to stop them. Which is all the more reason why we need to intercept the bad guys abroad and at home long before they are about to act—maybe even before they are sure they want to be terrorists at all. And to do that, the real challenge is figuring out what kind of explosive mixture of motivations is in their heads.

Using sociological analyses pioneered by the New York City Police Department and others, the Federal Bureau of Investigation and local law enforcement around the United States clearly are getting better at spotting people who want to "do jihad," as one alleged conspirator said while plotting to blow up a Bronx synagogue last spring.

"You have to understand the person," says a senior law-enforcement official who does not comment on the record about sensitive cases. "You have to get out in front of him [psychologically], make an assessment of whether he's radicalized enough to take action, and then make decisions about what you're going to do about it." The three highly publicized arrests of alleged wannabe bombers in Colorado, Illinois, and Texas over the last few days are all examples of the FBI, other federal agencies, and local law enforcement working along those lines to neutralize threats. In Dallas, the suspect was a troubled young Jordanian immigrant named Hosam Maher Husein Smadi who allegedly wanted to bomb a skyscraper. In Springfield, Ill., a convert to Islam named Michael Finton (a.k.a. Talib Islam) is charged with plotting to blow up the federal courthouse. In both cases (and also in the Bronx synagogue case) FBI stings let the suspects get to the point of planting what they believed were explosives, then the arrests were made.

The case of Najibullah Zazi in Colorado is a little more complicated. The young Afghan immigrant who went to high school in Queens, N.Y., is alleged to have had direct links to Al Qaeda in Pakistan and to have been taught bomb making there. The NYPD, the FBI, and other federal agencies are pursuing the possibility that he was part of a more extensive network, possibly including buddies from school in New York.

None of these cases has been tied to the others, but the key players appear to share the same mix of elements that have mingled in the minds of terrorists throughout modern history. And this is true whether they are leaderless jihadis improvising on their own or trained cadres answering to Osama bin Laden directly (as may have been the case with the belly bomber in Riyadh).