In New York, the damage continued into late afternoon, as a third building in the World Trade Center complex, Building 7, collapsed in a plume of flames at about 5:20 p.m. Authorities had been moving people out of the area prior to the collapse, as a fire in the lower part of the 47-floor building had made it unstable.
In the wake of the attacks, the U.S. Capitol, White House and other federal buildings were evacuated in Washington. In New York, U.N. headquarters was also evacuated, as were skyscrapers and federal buildings in several other cities. The Federal Aviation Administration ordered all airports around the country closed in the first such nationwide shutdown. The flight ban was not expected to be lifted until at least Wednesday at noon ET.
When the initial attacks occurred, the president was visiting an elementary school in Florida, but left after getting word of the disaster. He landed in Louisiana shortly before noon, then left on Air Force One for Nebraska and only headed back to Washington in the late afternoon after security officials deemed it safe for him to return, said ABCNEWS correspondent Ann Compton, who was with the president.
As the morning went on, and the horror of the initial crash grew with each new disaster, officials in Washington and New York moved to try to limit any further loss of life.
Police in Washington closed off a two-block perimeter around the White House and agents with automatic weapons and machine guns moved tourists out of the area, locking down entire buildings around the area.
Cell phones may have been deliberately turned off by servers because of fear the phones could be used to detonate a bomb.
In New York, Mayor Giuliani ordered that lower Manhattan be evacuated, asking everyone below Canal Street to walk north out of the area. The city's subway system was also shut down for hours.
An evacuation was ordered at the Sears Tower in Chicago, Los Angeles mobilized its anti-terrorism division, the Space Needle in Seattle was closed, and security was intensified around the naval facilities in Hampton Roads, Va. The financial markets in New York were closed through Wednesday, at least.
After the attacks, all military bases were put on Threatcon Delta, the highest level of alert. The FAA shut down all air traffic in the country at 9:25 a.m. ET, ordering any planes in the air to land at the nearest airport. Trans-Atlantic flights were rerouted.
In response to today's attacks, the U.S. Atlantic Fleet based in Norfolk, Va., sent ships to sea to contribute to the "air defense" of Washington and New York. Other ships were directed to get under way to provide humanitarian and medical support.
Law enforcement officials at the highest levels were stunned at the level and sophistication of the attacks, officials said, and have launched a massive probe. Although it is still too early to know the source of the attacks, the earliest theories focused on America's most-wanted fugitive.
U.S. officials familiar with terrorist activity say there is some information that points in the direction of Osama bin Laden, who tops the FBI's 10 Most Wanted list with a $5 million price on his head. He and his organization are high on the list of suspects, officials say. The attacks were too complex and well-coordinated for most terrorist groups to carry off, they said.