Umar Farouk Abdulmutallab, who tried to blow up a plane over Detroit on Christmas Day, was already a suspect before he boarded the plane in Lagos. The son of a former Nigerian minister and banker, Abdulmutallab was issued a visa at the U.S. consulate in London. But in November, after a call from his worried father, he was put on the Terrorist Identities Datamart Environment — a list containing as many as 550,000 names.

But that doesn’t mean he should have been denied boarding the flight to the U.S. at Schiphol airport. The student, who is from a wealthy background, said he operated for Al-Qaeda in Yemen. He was not registered on the “no fly list,” which contains 4,000 potential terrorists, nor was he on the list of 14,000 people who are subject to additional screening.

Schiphol Airport does not have a renowned image for security, which was partly revealed in an undercover report by Dutch TV-Channel SBS, in which a fake bomb was smuggled into an aircraft. Nevertheless, the relatively low degree of suspicion is probably why Abdulmutallab was not stopped and arrested. The explosives that he wore on his body would have been found by manual searches, which may have been performed if Abdulmutallab had been among the 18,000 names on those two lists.

These shortcomings are known. Eight years ago, in late 2001, “shoe bomber” Richard Reid was on a flight en route from Paris to Miami and carried the same explosive chemicals as Abdulmutallab. Since then, security at many airports after 9/11 has become even more stringent. More and more passengers must take off their shoes, and liquids cannot be taken on board. But these procedures do not guarantee watertight security — certainly not when a loner conceives of a plan to commit a terrorist act, or when someone behaves suspiciously on a plane like Sunday’s flight from Amsterdam to Detroit.

President Obama has called for new security measures, and the Netherlands should follow this example. Although Schiphol has followed protocol, research into new security measures can correct ineffective disciplines and fill in procedural gaps.

Research could also shed light on whether the offender was part of a network or operated on his own. The answer is politically sensitive in both the Netherlands and the United States, but it may provide a clear view of the terrorist threat in order to keep an eye on the extent of countermeasures.

100 percent certainty cannot be attained. Terrorists, especially if they want to take their own lives, are always ahead of the security. Risks can be reduced, not eliminated. Total security is an illusion in an open democratic society.