On Monday evening, Dutch authorities detained Ahmed Mohamed Nasser al Soofi (of Detroit) and Hezem al Murisi when their United Airlines flight from Chicago landed in Amsterdam. It’s always a big mistake to jump to “instant analysis” based on preliminary press reports. But it shouldn’t come as much of a surprise if these two men turn out to have had bad intent.

Al Soofi and al Murisi are suspected of making a dry run for a future terrorist attack. Dry runs are common in the terrorism trade. Terrorists are a relatively scarce asset. They like predictability. They like to know the security they’ll be facing and the likelihood they can pull off a successful operation.

Nor is it surprising to find terrorists departing from the United States rather than heading to it. It has been all the fashion lately. David Coleman Headley, a Chicago-based Pakistani-American, was dispatched from the U.S. to scout out terrorist targets in Mumbai. In June, two New Jersey men — Mohamed Mahmood Alessa and Carlos Eduardo Almont — were arrested at JFK as they were allegedly heading to join al Shabaab, an al-Qaeda affiliate in Somalia.

According to news reports, one of al Soofi’s neighbors says he was from Yemen. Apparently, he had booked to a flight to Yemen as well. A Yemeni connection wouldn’t be surprising. Al-Qaeda in the Arabian Peninsula, a group based in Yemen, dispatched the Detroit-bound Christmas bomber Umar Farouk Abdulmutallab. Not only is this group affiliated with al-Qaeda, they have a network in the U.S. and have openly declared their intent to attack America and American interests.

It would surprise no one to find a terrorist attack aimed at killing Americans. Since 9/11, authorities have thwarted at least 31 terrorist attacks and plots aimed at U.S. citizens. Some of those involved attacking commercial aviation. In addition to Abdulmutallab’s aborted attempt, a foiled 2006 plot aimed to blow up international flights bound for the U.S. using liquid explosives.

With today’s arrest, speculation will focus on just what the two suspects were really up to and why they were allowed to board an international flight to begin with. TSA agents had deemed the pair “suspicious.” Al Soofi was even stopped and searched in Birmingham, Alabama, where the flight originated.

The stop is a good thing. Authorities pulled him aside because he wore suspiciously bulky clothing — a trademark of a mule (i.e., someone secretly bringing aboard something they shouldn’t). Oh, and baggage searches revealed a “mobile phone taped to a Pepto-Bismol bottle, three mobile phones taped together and several watches taped together.” That’s a tad out of the ordinary, too. More curious still, al Soofi booked his luggage on a flight to Dulles and then on to Dubai and Yemen, while himself on a flight to Amsterdam. That will raise a few eyebrows among thesecurity-conscious.

Perhaps officials had reason to let them continue to travel. Perhaps, the feds made sure there was an air marshal — or two, or three — aboard the flight with the two “suspicious” travelers.

At this point, it’s too soon to criticize aviation security or to ship the suspect off to prison. More facts are needed.

But this incident should remind us of three indisputable facts: 1) There are terrorists out there; 2) they are trying to kill us; and 3) if we don’t work proactively to stop them, one day they will succeed again.

Cross-posted at The Corner.