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.
Tighter Scrutiny in the Air
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.
The Holes We Know
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.
One backpack may seem innocuous, but with 15 pounds of explosives inside it, it can do devastating damage.
Also last year, ABC News found a cargo train with tankers filled with chlorine and other hazardous materials stopped near a federal office building housing 71,000 workers. The tankers were left there -- vulnerable --- for nearly an hour.
"It would take a terrorist about five seconds' access to a railcar to cause a real catastrophe," said Homeland Security consultant Randal Larsen.
And in 2002 and again in 2003, ABC's investigative unit showed how easy it was to smuggle uranium into the United States by cargo ship.
Senior investigative correspondent Brian Ross and his producers shipped depleted uranium -- which is legal -- but it still should have triggered an inspection and been detected by customs officials. It was not.
So while there were more police, more bomb-sniffing dogs and a heightened awareness across the country today, does that mean we are safer?
We don't even have any idea when, where or how the next attack might come.
Experts say the terrorists are experiencing a paradigm shift that began with the attacks in Madrid. Their plan used to be to attack the government; blowing up embassies, flying planes into the Pentagon or the White House or the Capitol.
The new paradigm, experts said, is to attack the citizenry -- the working man and woman. They're trying to frighten people so that they will pressure their government.
However, Miller asserted that people shouldn't be afraid -- "because if we wake up in the morning and are afraid, then the terrorists have won."
In New York, Gov. George Pataki, a Republican, echoed that thought. "So take the subway. Go to the parks . The best security in the world is there to protect you."