The arrests of 21 UK-based suspects by British authorities, plotters who were allegedly trying to blow up multiple aircraft, tells me that al-Qa'ida is still using its tried and true operational template of taking time to surveil, probe, and recon its targets to seek the weak points before it commits to an attack.

The 21 suspects arrested on 10 August are only the tip of the iceberg. The logistics of multiple aircraft bombings are hugely complicated. Indeed, US intelligence learned just how complicated during the investigation of the 1995 “Operation Bojinka,” in which World Trade Center bomber Ramzi Yusef and al-Qa'ida operations chief Khalid Sheikh Mohammed planned to bring down a minimum of a dozen U.S. aircraft over the Pacific. Covert terrorist operatives must scope out each flight -- just as the 9/11 terrorists did -- to observe where the weak spots lie. Airports must be cased, their strengths and weaknesses noted, and the plot tailored to ensure the best chance of success. If one piece of the target is hardened, al-Qa'ida's MO is to probe until it finds a soft spot.

Post 9/11, for example, federal authorities checked hand baggage, confiscating knitting needles, pocketknives, cigarette lighters, and corkscrews. But didn't bother with anybody's shoes.

Enter Richard Reid, stage left.

Reid, the British al-Qa'ida shoe-bomber, was only caught because he took too much time trying to light the fuse on his PETN-lined shoes. Immediately thereafter, anyone flying anywhere had to remove their shoes for inspection. Which meant that al-Qa'ida had to mutate, transmogrifying its modus operandi to seek other weak-points.

Like beverages. Liquid-based explosives were considered by Khalid Sheikh Mohammed for Operation Bojinka back in 1995. But since then, we seem to have been lulled into a false sense of security when it comes to carrying food and drink onto an aircraft. Everybody does it.

Which is the problem.

Why? Because beverage-weapons have always had great potential fro terrorists. I have a friend named Lou who has spent decades thwarting terrorist attacks and teaching others how to harden potential targets. Lou is a tremendously effective counterterrorist because he thinks like a bad guy. One of his better training gimmicks was to cut open a soft drink can, slip a dummy grenade inside, then put the can back together and refill it with soda. He'd be drinking as he walked through the airport metal detector. When the machine was set off he'd look at his can, hand it to one of the agents, walk through again, take the can and sip his drink. No security agent ever caught on. Today, the grenade in the soda can gambit might work in the Third or Fourth world, but I don't think anyone would try it at Heathrow or Dulles.

But there are other ways. I've flown in and out of both Heathrow and De Gaulle a few times over the last few months and I've noticed huge gaps in airport security in each airport. Sure, Heathrow has hundreds of armed police and plainclothes officers on duty, and at De Gaulle there are spot checks of hand baggage as well as sniffers for cargo going into the plane's hold.

But it's been five years since 9/11, and operating on full alert 24/7 is tough. Passengers hate inconvenience -- and they're vocal about it. The sheer tedium of operating the X‑Ray machines hour after hour becomes overwhelming. Yes, the searches do identify forbidden items -- scissors, pocketknives and the like. But nobody ever questions a Starbucks Grande, or a bottle of Frappacino these days. Nobody blinks at an iPod, a laptop, a portable DVD player, a PDA, or a cell phone anymore. From my eyes-on experience, cursory looks and quick scans are what these potential weapons receive from British, French, and American security personnel.

But here's a fact: Bomb-making has come a long way in the past five years. The bad guys can miniaturize and compartmentalize weapons as well as anyone. A PDA can become a detonator and still operate as a PDA. Select liquids can be used to mask any trace of explosives. Other liquids or gels can actually be used as a transporting agent for explosives. Thick, sweet drinks might be used to camouflage small amounts of Semtex plastique explosive, which could be assembled onboard the aircraft.

Then there's the problem of sleepers. The PLO, for example, used to run many of the duty free shops in sub-Saharan Africa. That gave Yasser Arafat's terrorists a huge leg up in smuggling weapons and other contraband from airport to airport, because they were able to penetrate the most secure and supposedly sterile areas of airports in Kenya, Uganda, and other locations.

Okay, how many al-Qa'ida sleepers are working as you read this column at De Gaulle and Heathrow, or Dulles, JFK, or LAX? How good are the security checks on the tens of thousands of employees who have unfettered access to the flight lines and the secured areas? What's the chance of an al-Qa'ida sleeper who works as part of a plane-cleaning crew being able to slip an explosive device into a plane's bathroom, where a Jihadi martyr passenger could activate it?

It is known, for example, that sleepers from MS-13 and the 18 h Street Gang, two of the most dangerous Salvadoran criminal organizations, have enlisted in the U.S. Armed Forces in order to learn organizational, combat, and intelligence techniques that can be used by the gangs. Should we think al-Qa'ida's lone-wolf operatives haven't tried -- and perhaps succeeded -- in doing the same thing both here and abroad?

As the British peel back the layers of the current plot, we have to remember that our adversaries are already watching to see what we do and how we do it. They will then reconfigure their attack plans to capitalize on the weakest links in our defenses. They are patient, cunning, and deadly. We must be ruthless, nimble, and proactive if we expect to disrupt their plans.