Every day, millions of Americans visit malls, ride trains and enter office buildings. Checking every one of them would be impossible, so a terrorist who's willing to die could detonate a bomb and kill many people virtually any time.

So why hasn't that happened?

Since the Sept. 11 attacks -- contrary to all expectations -- there have been no terrorist strikes on U.S. soil. And it's not because our enemies have stopped trying. A recent FBI report says al Qaeda hopes to attack shopping centers during the holidays. The terrorist group hopes "to disrupt the U.S. economy and has been planning the attack for the past two years."

The reason we haven't been hit is because U.S. authorities are actively pursuing -- and stopping -- individuals and groups intent on killing Americans.

This isn't always easy. Effective operations often require federal, state, local and even international cooperation. But our law enforcement agencies have already racked up an impressive string of arrests.

Perhaps the biggest fish caught so far is Khalid Sheikh Mohammed. He's tied to a number of terrorist plots, including Sept. 11, the 2002 Bali bombings and the 1993 bombing at the World Trade Center.

Undoubtedly one of Osama bin Laden's highest-ranking operatives, he also claims to have personally decapitated journalist Daniel Pearl. Under interrogation, KSM gave up the names of several fellow operatives, thus helping to prevent other attacks. He's being held at the U.S. military detention facility at Guantanamo Bay, awaiting trial by a military commission authorized by Congress.

In addition, we've captured many would-be terrorists right here in the U.S.

In September 2002, federal authorities arrested "the Lackawanna Six" near Buffalo, N.Y. All were American citizens; five had been born and raised here. All six pled guilty in 2003 to providing support to al-Qaeda and are now serving prison sentences.

Also prevented: an attack on a New York City subway train, planned for August 2004 (during the Republican national convention). James Elshafay and Shahawar Matin Siraj were detained after an undercover agent infiltrated their group. Both are in prison.

More recently, six men were arrested last May and charged with plotting to attack Fort Dix in New Jersey. They allegedly planned to ambush soldiers with assault rifles and grenades. Five of the suspects had trained for the mission, and the sixth helped get them weapons. The arrests were made after a 16-month Fbi operation, which included infiltrating the group. All the suspects are in custody awaiting trial.

Airports remain a popular target. This year, four men allegedly plotted to blow up a jet fuel pipeline near JFK airport in New York. One was a former airport worker, and two others had links to Islamic extremists overseas. This was a true transnational operation. The leader is being detained here in the U.S., while two others are being held abroad.

Then, just this month, authorities broke up a crime ring involving illegal aliens at O'Hare airport in Chicago. A raid netted 24 people accused of using fake security badges to work in restricted areas.

Our many successes prove that the best way to prevent terrorist attacks is through effective intelligence collection, information sharing and coordinated, determined counterterrorism operations that stop criminals before they can attack. Terrorist organizations haven't given up. They're still active, as the list of publicly-known arrests attests.

The best way to protect ourselves against transnational terrorism is to follow the example set by these impressive post-9/11 operations. We must respect the rule of law and protect our cherished liberties while also conducting active investigations to block terrorist conspiracies before they come to fruition.

We must defend freedom and remain vigilant. That's the only way to protect Americans during the long war on terrorism.

Ed Feulner is president of The Heritage Foundation (heritage.org).