As a horrified nation watched on television, the twin towers of the World Trade Center in lower Manhattan collapsed into flaming rubble after two Boeing 767s rammed their upper stories. A third airliner, a Boeing 757, flattened one of the Pentagon's five sides.
A fourth jetliner crashed in western Pennsylvania. Authorities said the hijackers might have been trying to aim the plane at the presidential retreat at Camp David, Md., the Capitol or other targets in Washington.
PHOTOS: The attacks of September, 11, 2001
The assaults, which stirred fear and anxiety across the country and evoked comparisons to Pearl Harbor, were carefully planned and coordinated, occurring within 50 minutes. No one claimed responsibility, but official suspicion quickly fell on Saudi fugitive Osama bin Laden. Unexplained was how the terrorists boarded the jets and overpowered the crews.
Federal law enforcement sources said the FBI conducted searches and served subpoenas, some in south Florida. One official said agents were investigating the possibility that some of the terrorists were pilots who had been trained "for this kind of action."
The FBI was sifting through hundreds of tips pouring into a toll-free hotline and a Web site and pursuing dozens of leads.
Addressing the nation Tuesday night, President Bush vowed to "find those responsible and bring them to justice." This country, he said, would retaliate against "those behind these evil acts" and any country that harbors them.
Altogether, the four downed planes carried 266 people. All were killed. Scores of people jumped to their deaths or died in fires and the collapsing superstructure at the Trade Center. New York Mayor Rudolph W. Giuliani said earliest reports counted 2,100 people injured, about 150 of them in critical condition.
Estimates of the death toll at the Pentagon ranged from 100 to 800.
At nightfall, more than nine hours after the attack, a 47-story annex to the 110-floor twin towers at the Trade Center collapsed as well. It too had caught fire, but by the time it fell, all of its occupants had been evacuated.
At a late-night news conference, New York authorities said more than 300 firefighters and three dozen police officers were missing. Many had rushed into the towers after the airliners hit, only to be trapped when the buildings collapsed.
Among the dead were the New York fire chief, his chief of special operations and a first deputy commissioner.
Some of those still in the rubble reportedly called officials or family members on their cell phones, and some trapped police officers made radio contact with headquarters. But because of fires and unstable debris, rescue attempts were halted after dark.
Bush placed U.S. forces around the world on highest alert and flew from a visit in Florida to secure military bases in Louisiana and Nebraska before returning to Washington to address the nation. Vice President Dick Cheney, Cabinet members, congressional leaders, and the president's family also were taken to secure locations.
It was the worst siege of terrorism waged against the United States in its history. It shut down the federal government in Washington and the financial markets in New York. It closed all airports across the nation for the first time, as well as some Amtrak rail lines in the Northeast. It put off the primary election in New York and closed Disneyland. It halted major league baseball for a day, as only World War I and D-day have done before.
America tightened security at its borders and at embassies and military sites around the world. The National Guard patrolled Washington and New York. Bridges and tunnels into Manhattan were closed. Authorities evacuated the Capitol Building, the State Department, the CIA building, the United Nations and the Sears Tower in Chicago. Hoover Dam was closed to visitors. Patrols increased along the trans-Alaska oil pipeline.
Even in Europe, authorities evacuated high-rise buildings as a precaution.
Members of Congress, after being briefed by FBI and intelligence officials, said Bin Laden was the suspected mastermind. "They've come to the conclusion," said Sen. Orrin G. Hatch (R-Utah), "that this has the signature of Osama bin Laden." He is the fugitive Saudi terrorist under indictment here for the bombing of American embassies in Kenya and Tanzania. Bin Laden has been granted asylum by Afghanistan.