NEW YORK — In the worst terrorist attack in U.S. history, hijackers flew two airliners into the World Trade Center today, collapsing both towers into flaming rubble, and crashed another aircraft at the Pentagon, shutting down the government and financial markets and spreading fear throughout America.
The toll of dead or injured was expected to climb into the thousands.
Hours later, a fourth airliner, bound from Newark, N.J., to San Francisco, went down in western Pennsylvania. Authorities were not immediately certain whether this crash was caused by terrorists.
In addition, a major airline said a plane, bound from Boston to Los Angeles, had gone down at a location it did not immediately disclose. Authorities would not say whether this was the aircraft that hit the Pentagon.
For the first time ever, the Federal Aviation Administration closed all airports across the nation and ordered U.S.-based airlines to cancel all domestic and international flights until at least noon EDT Wednesday. European aircraft were diverted to airports in Canada. Amtrak suspended service along the Northeast corridor.
"This is perhaps the most audacious terrorist attack that's ever taken place in the world," said Chris Yates, an aviation expert at Jane's Transport magazine in London. "It takes a logistics operation from the terror group involved that is second to none. Only a very small handful of terror groups is on that list. . . . I would name at the top of the list Osama bin Laden."
President Bush ordered federal investigators to "hunt down the folks who committed this act." The Army deployed troops in Washington, including an infantry regiment. The Situation Room at the White House went into full operation. Authorities went on alert from coast to coast, the U.S. borders with Canada and Mexico were sealed, and security was tightened at strategic installations.
The Federal Reserve, seeking to provide assurances that the nation's banking system would be protected, said it stood ready to provide additional money to banks if needed.
The nation huddled in shock and mourning. Events across the country were canceled. Major league baseball postponed its daily schedule of 15 games.
"This is the second Pearl Harbor," Sen. Charles Hagel (R-Neb.) told reporters in Washington. "I don't think I overstate it."
The terror began shortly before 8 a.m. EDT when American Airlines Flight 11, a Boeing 767 carrying 81 passengers, nine flight attendants and two pilots, was hijacked shortly after leaving Boston for Los Angeles. It flew south to New York, swept to the bottom of Manhattan island and flew squarely into the north tower of the World Trade Center.
Minutes later, American Flight 77, a Boeing 757 carrying 58 passengers, four flight attendants and two pilots, was hijacked shortly after leaving Dulles International Airport in Washington for Los Angeles. It flew north to New York, reached the bottom of Manhattan and rammed directly into the trade center's south tower.
"I heard a plane fly overhead," said Park Foreman, 37, an Internet security consultant. "Then I looked out the window and saw the first tower on fire. I saw another airplane approaching from the south. I put my camera on it and followed it straight into the building. It looked like it went right through.
"A huge fireball erupted from all four sides. Three seconds later, I felt the shock wave. My heart just went into my stomach.
"I saw the second tower fall. I saw it collapse. It was the scariest thing I've ever seen in my life. You could see people jumping out of the tower on fire. That moment changed my life."
Shortly afterward, the Pentagon took a direct hit. The nation's military nerve center exploded into flames and part of one of its five sides collapsed. Secondary explosions sent smoke billowing across the Potomac River and into Washington. Glenn Flood, a Pentagon spokesman, said there were "extensive casualties and an unknown number of fatalities."
The Pentagon and the Capitol were evacuated, as were other federal buildings in Washington and New York. The president was in Florida at the time. Vice President Dick Cheney was in Washington, and he and First Lady Laura Bush were taken to a secret, secure location.
Unexplained blasts were reported near the State Department and in the vicinity of the Capitol.
In Pennsylvania, United Airlines Flight 93, a Boeing 757 carrying 38 passengers, two pilots and five flight attendants from Newark International Airport to San Francisco, crashed about 80 miles southeast of Pittsburgh. There was no immediate word of survivors.
The airline said United Flight 175, a Boeing 767 bound from Boston to Los Angeles with 56 passengers, two pilots and seven flight attendants, also went down. Neither the airline nor authorities would say where it crashed.
Afghanistan's hard-line Islamic Taliban rulers condemned the attacks and rejected suggestions that bin Laden was behind them. Bin Laden has been given asylum in Afghanistan.