Timeline of U.S. Attacks in NYC, D.C.

— Plane crashes into tower of World Trade Center in lower Manhattan, shortly before 9 a.m. Eastern.

— Second plane crashes into the second tower of the World Trade Center, shortly after 9 a.m. Eastern.

— President Bush, in Sarasota, Florida, calls the crashes "an apparent terrorist attack" and a "national tragedy."

— An aircraft crashes near Pentagon, just outside of Washington D.C., in Northern Virginia, about an hour after the attacks in New York.

— Government buildings in Washington, including the Capitol and the White House, are evacuated with officials citing a credible threat of a terrorist attack.

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— The Federal Aviation Administration shuts down all aircraft takeoffs nationwide.

— Shortly after 10 a.m. Eastern, one World Trade Center tower in New York collapses, about an hour after being hit by plane.

— American Airlines says one of the planes that crashed into the Trade Center was American Airlines Flight 11, hijacked after takeoff from Boston en route to Los Angeles.

— Senior law enforcement officials say car bomb explodes outside of State Department in Washington, D.C. Federal protective services later denies car bomb attack occurred.

— Financial markets suspend trading in the wake of the attacks.

— Officials at Somerset County Airport say a large plane crashes in western Pennsylvania, about 80 miles southeast of Pittsburgh, at about 10 a.m.

— The second tower of the World Trade Center collapses at 10:28 a.m. Eastern.

— Fourth explosion rocks the collapsed remains of the World Trade Center, at about 10:38 a.m.

— Authorities across the country go on alert, tightening security at strategic facilities and evacuating high-profile buildings. U-S monuments and museums in Washington D.C. are closed.

— Securities and Exchange Commission says all financial markets are closed for the day.