Eight years ago today the United States was attacked in what was the worst terrorist attack in the nation's history. Since that day America's leaders have been warning the country that an attack on U.S. soil is imminent.

In January 2002, during President Bush's first State of the Union address, he said "Thousands of dangerous killers, schooled in the methods of murder, often supported by outlaw regimes, are now spread throughout the world like ticking time bombs, set to go off without warning."

Vice President Cheney followed the president's warning a few months later in May saying, "I think that the prospects of a future attack on the U.S. are almost a certainty. It could happen tomorrow, it could happen next week, it could happen next year, but they will keep trying."

America watched nervously, anticipating the worst but no attack came. Since that sunny September morning 8 years ago the United States has been able to stay safe – but how?

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Tomorrow is September 11 – an important time to ask tough questions like this one: Could terrorists get their hands on a weapon that could do even more damage and smuggle it into the US?

CNN's Paula Newton reports in part two of our special series, "Spies Among Us."

The FBI ranks China as one of the biggest espionage threats to the United States over the next decade.

From top secret research to items up for sale on eBay – some of our most intimate national security details could be at risk. Homeland Security Correspondent Jeanne Meserve has the first part of our special series 'Spies Among Us.'