WASHINGTON — Two years after the Sept. 11 attacks, the Muslim extremists who turned four commercial aircraft into deadly missiles haven't managed to strike again on American soil.

Government officials and terrorism experts say this is due to stepped-up security measures and greater vigilance by federal and local law enforcement agencies.

But in the same breath, they admit that al-Qaida sympathizers are living in America and the absence of more attacks here is a measure of luck.

"I think we've been blessed. I think we've been lucky that an attack hasn't happened here, but we have to make our own luck," said Deputy Attorney General Larry Thompson, in an interview. "We have daunting odds against us, but we have to be successful."

To bolster defenses against attacks, the government reorganized itself with the creation of the Homeland Security Department, combining nearly 2 dozen agencies and 170,000 employees. The FBI, criticized for being more concerned about solving crimes than preventing them, has sharply increased counterterrorism efforts. An interagency Foreign Terrorist Tracking Task Force has been created, and intelligence agencies say they're doing a better job sharing information.

Passenger screening at airports is more thorough, cockpit doors have been hardened and passengers are more vigilant — all of which makes it less likely that the Sept. 11 plot could be repeated today.

But terrorists can strike in many other ways.

The next attack could target chemical or nuclear plants. Or municipal water systems. Anthrax or sarin could contaminate large office buildings or subways. Waves of suicide bombers could target crowds in major cities. And any number of these attacks could occur simultaneously, overwhelming police, paramedics and hospitals.

"Two smart people in a room for two hours can come up with 100 really scary things," said Daniel Byman, a terrorism analyst at the Brookings Institution and a former staff member on the joint House-Senate inquiry into the Sept. 11 attacks.

If the war on terrorism stretched across a baseball season, then the FBI, CIA and the rest of the nation's intelligence-gathering agencies would have to pitch a no-hitter every game to win, Thompson said.

"All a terrorist has to do to be successful is to get a single in one game," Thompson said.

One bomb. One attack. One incident where more innocent lives are shed on American streets is all it would take and "the game is over," Thompson said.