YON, France -- While the important work of rescuing and treating the many victims of Tuesday's terrorist crimes continues, people are asking: How could this have happened? Was there a way to prevent such a crime?

Every country is vulnerable to suicidal terrorist acts. In 1995, a White House security review, undertaken after a plane crashed onto the White House south lawn, examined the strategies of many countries to protect their heads of state. Although the situation isn't exactly the same as the attacks in Washington and New York, there are lessons to be learned. How could one stop a terrorist attack like the one that happened this week?

The options are few. We could say that we will shoot down a commercial or private aircraft that violates a no-fly zone in densely populated areas. In reality, it is hard to imagine that we would ever shoot down civilian aircraft.

We could try much harder to make airports invulnerable to terrorists. The Federal Aviation Administration has now banned all knives on board and stopped curbside check-in of luggage. While helpful, these measures will not cure the problem. We could try to establish a perimeter within airports where everyone is searched. But any frequent traveler knows that private security officers who inspect passengers and luggage in American airports are generally undertrained, underpaid, undermotivated and ineffective. Even if we placed federal agents in these positions, the monotony of the job is likely to lead to errors. Weapons can be easily disguised, concealed and fashioned from just about anything. Guns are made to look like belt buckles, pens, cigarette packs and key chains.

We could also do more to secure the cockpit. Before September 11, 2001, we were mostly concerned with what hijackers would do to passengers on a plane; now we must worry about the plane being used as a weapon. We may have to consider building a safe zone between the cockpit and the passengers, so that pilots can exit the cockpit and secure the door, then enter the safe zone to eat and sleep.

This safe zone isn't foolproof, however. It's hard to imagine that pilots could be trained to act like armored- car drivers who are ordered to never open the door, no matter the circumstances. For instance, what if passengers are being murdered to force the pilots to open the cockpit door?

While we must increase airport and airplane security, a completely fail-proof system — one that balances safety and efficiency — would be hard to imagine. National security officials tend to focus on covert intelligence operations, and those activities, of course, are central to fighting terrorism. But we must also improve the ways we collect criminal intelligence through international police forces.

Prevention of terrorism can be executed well only if law enforcement becomes more global in reach, to match terrorism's global reach. The best information often comes from neighbors that see unusual things near their homes or businesses, and this information is ordinarily given to local police officers, who then are in position to relay the information to national or international police. It's a chain reaction that begins at the local level.

Admittedly, these police may not be as sophisticated as America's, but they can be trained to pick up vital clues. Unfortunately, the United States and other wealthy countries do not devote enough resources to ensure that not-so-wealthy countries have a way to collect, analyze and transmit police intelligence. These overseas police forces are either not trusted or looked down upon. And it is difficult for politicians to explain how expenditures internationally reduce crime back home.

It's fine to talk about fighting terrorism, but the United States and other countries must invest more in law enforcement agencies outside their own countries. Investing in the world's police forces and Interpol is the only way to ensure that valuable intelligence can be gathered, analyzed and shared internationally. Police officers worldwide must be properly equipped, trained and motivated to stop terrorism.