BRUSSELS, Belgium — Here in Europe, people woke yesterday to the gradually unfolding news that police had thwarted a terrorist plot to plant explosive devices aboard flights from London's Heathrow airport to the United States.

Americans had a rather ruder awakening, learning over morning coffee even fuller details of this latest terrorist threat and news that the national alert status had been heightened to its highest level — "red" (severe) — for flights from Britain to the United States, and to the second highest level — "orange" (high) — for all other civilian domestic and international flights.

As of this writing, details of the bomb plot that closed the world's busiest single-runway airport to incoming flights, throwing it into chaos, are still emerging.

According to early reports, planes from three U.S. airlines — United, American and Continental — were to be the intended targets. British authorities have detained 24 suspects and are carrying out follow-up searches. Meanwhile, British authorities also raised their national threat level to its highest level — critical — "indicating that an attack is expected imminently."

While the number, destination and timing of the attacks are still under investigation, some reports say that a series of three terrorist strikes were being planned, each targeting three aircraft. The nature of security countermeasures indicate that the individual components of these devices (which may have used liquid explosive charges) were to be smuggled aboard in individual carry-on luggage, with the bombs being assembled and detonated when the aircraft were airborne.

If these reports are accurate, the plot could potentially have resulted in more casualties than the Sept. 11 terrorist attacks on America in 2001. This is a sobering consideration, as are reports that many of those arrested are said to be British citizens. As with the July 7, 2005, attacks in London, this highlights the home-grown aspect of the terrorist threat, even as the disruption caused by yesterday's alert and the nature of the alleged plot and its targets emphasizes its international dimension.

As for propaganda, the apparent breadth of the would-be bombers' vision will resonate more deeply with their supporters than the fact that their plot appears to have been foiled. Moreover, even a foiled terrorist attack can cost dearly.

It is comforting that, thus far, the cost has been relatively low (more an inconvenience than a loss of blood and treasure) compared to the cost that would have been incurred had the terrorists been successful. But the physiological and economic costs of yesterday's events will not be insignificant.

While it is still too early to gauge the long-term fears these events will have invoked among the public, the economic fallout already has begun to manifest itself: share prices in the airline and travel industry are falling. Even if passenger and shareholder confidence bounces back quickly, the costs of the security response, flight delays and any eventual compensation to travelers shows that, for the terrorists, their modus operandi retains a healthy cost/benefit ratio.

Yesterday's events also underline the international dimension of the terrorist threat. Even if all the Heathrow plotters turn out to be British — an extremely worrying development for that country and all others with similarly susceptible citizens — they doubtless will have been motivated and inspired by events and individuals far from Britain.

Moreover, the fact that major airports like Heathrow are regional and global chokepoints that are as vulnerable to the credible threat of terrorist attack as to actual terrorist attack serves to remind us of the international nature of much of today's terrorism. This is true even if those perpetrating the attacks often turn out to be homegrown. Not all terrorism is international, but the line between international terrorism and its domestic variant is increasingly blurred.

As the fifth anniversary of the Sept. 11 attacks draws near, the degree to which the Heathrow plot is verifiably demonstrated to have been a credible one, and just how likely it was to succeed, will be crucial in assessing terrorist capabilities.

Their intent is not in question.

Mark Burgess is the director of the Brussels office of the World Security Institute in Washington, D.C. He wrote this commentary for Global Beat Syndicate.