Could a London-Style Attack Happen Here?

July 8, 2005 — -- Law enforcement officials across the United States acted swiftly to increase security in their cities in the wake of the terrorist bombings in London.

But John Miller, a former ABC News investigative reporter and now head of the Los Angeles Police Department's counterterrorism bureau, says it's never possible to protect everything.

"There is an old Chinese proverb: 'He who protects everything protects nothing,'" he said.

Protecting travelers in the United States is a daunting task. Twenty-nine million people commute every day on trains, subways and buses. There are 600,000 bridges and tunnels in America. Roads extend nearly 4 million miles.

After the attacks on the World Trade Center and the Pentagon on Sept. 11, 2001, the federal government moved aggressively to protect air commuters.

But little has been done to make those who travel on the ground safer, according to Robert Puentes of the Brookings Institution.

The kind of security that has become commonplace at the nation's airports, like improved screening and air marshals on flights, is not practical for mass transit, Puente conceded. What happened in London can happen here, he said.

Miller said it would be impossible to put metal detectors on trains tomorrow, but if a terrorist attack occurs, it is a possibility.

In fact, nearly half of all terrorist attacks in the past decade have occurred on buses or trains because they are so hard to secure.

In last year's terror attack on Moscow's public transit system, 41 people died. A month later, the attacks on railways in Madrid killed nearly 200.

Since 9/11, ABC News has conducted several tests to see how vulnerable the U.S. transportation system is, and exposed some gaping holes.

Last year, producers left a backpack in plain view on a commuter train in Maryland. No one noticed it for 45 minutes. Another backpack traveled two-and-a-half hours from Washington, all the way to New York City's Penn Station with no one doing a thing.