专家:美国自9-11事件后更安全

Security experts say Americans are much safer from terrorism since the attacks on September 11, 2001 that killed nearly 3,000 people. However, they are quick to add the deadly bombings at the Boston Marathon underscore the difficulty of protecting people against a determined attacker targeting civilians.

Following the September 11, the country’s security professionals made enormous strides in protecting civilians. Dozens of plots have been prevented, usually before there is danger to the public.

“If the question is, is America a harder target for transnational terrorism than it was before 9-11, the answer is you bet,” said James Carafano, a security expert at The Heritage Foundation. “Most of it has been based on good intelligence, good proactive counterterrorism efforts, finding people who are interested in doing violence, getting inside, getting information about them and taking them down before they are a threat.”

Carafano credits the success in part to the FBI's shift from a national law enforcement agency to an organization that made counterterrorism its top priority.

He also says the Department of Homeland Security has become skilled at securing airports and the nation’s borders.

But the Boston bombings show how difficult it is to protect the public from all attacks.

“I think you have to accept that this kind of an attack sometimes is going to be possible. And we have had a number of cases in recent years where attackers almost succeeded,” stated Michael O’Hanlon, senior fellow at the Brookings Institution.

The last major terror attack occurred when Army Major Nidal Hassan allegedly killed 13 people at Fort Hood, Texas in 2009. He had connections to al-Qaida in the Arabian Peninsula.

But a number of bomb plots have failed or have been foiled.

Several months after the 9-11 attacks, Richard Reid failed to detonate a shoe bomb on an airliner.

And on Christmas day 2009, the so-called "underwear bomber," Umar Farouk Abdulmutallab, failed to ignite explosives on a flight to Detroit.

That year authorities arrested an Afghan immigrant, Najibullah Zazi, plotting to bomb the New York subway.

Three years ago, New York's Times Square was evacuated after the discovery of a car bomb.

While analysts say officials have learned how to protect people in large public gatherings, there's a limit to what is practical and effective.

“They are not going to necessarily stop a very determined attacker," Carafano said. "You really have to rely on finding that person and stopping them before they get there.

As counterterrorism officials say -- they have to be perfect all the time, while terrorists have to succeed only once.