The terrorists' audacity was matched by the stunning coordination of their operation - all four planes had left three airports within 12 minutes of one another - and experts scrambled to start investigating how they had bypassed security and pulled it off. U.S. officials said they had had no reports that the attacks were imminent and can expect questions about the breakdown in intelligence.
The carnage seemed destined to stand as the worst attack on civilians in U.S. history. The death toll - which could take days, even weeks, to emerge - was likely to be far more catastrophic than the 2,400 killed nearly 60 years ago in the surprise bombing at Pearl Harbor.
Emergency workers at the World Trade Center site in Lower Manhattan faced a gruesome, blazing scene.
Some people were alive in the debris and were making cell-phone calls, New York Mayor Rudolph Giuliani said last night, but workers continued to be hampered by flames from the wrecked buildings.
"When we get the final number [of fatalities], it will be more than we can bear," Giuliani said.
Between 40,000 and 50,000 people worked at the World Trade Center complex, 20,000 of them in the towers; an additional 90,000 visited on an average day. At the Pentagon, officials told one congressman that the building appeared to have sustained about 100 casualties. The four hijacked flights - two from American Airlines, two from United - carried 266 people in all.
President Bush, preceded by a string of cabinet officers and congressional leaders, took to the airwaves last night to try to calm the nation and vow justice for the perpetrators. He put U.S. military forces around the world on an alert of the highest level.
"We will make no distinction between the terrorists who committed these acts and those who harbor them," Bush declared in a televised address from the White House, where he had returned late in the day after taking a circuitous route from Florida to Louisiana and Nebraska for security reasons.
No groups claimed responsibility. Afghanistan's hard-line Taliban rulers condemned the attacks and rejected U.S. officials' suggestions that terrorist mastermind Osama bin Laden might be behind them.
The United States says bin Laden, who is believed to be hiding in Afghanistan, has organized and financed numerous terrorist operations. Sen. Orrin G. Hatch (R., Utah), the top Republican on the Senate Judiciary Committee, said U.S. intelligence had intercepted communications between bin Laden supporters discussing the attacks.
Explosions were heard early today near the airport in Afghanistan's capital of Kabul, but the United States denied involvement. One U.S. official said the fighting appeared to have been rocket attacks by rebels.
Army Gen. Henry Shelton, chairman of the Joint Chiefs of Staff, would not talk publicly about possible U.S. military retaliation.
"I have no intention of discussing what comes next, but, make no mistake, our armed forces are ready," he said.
As the panic and chaos unfolded in New York and Washington, the nation went on high alert, and normally bustling centers of activity emptied out. Federal agencies, schools, malls, landmarks and tourist sites, including Independence Hall, shut down. All U.S. financial markets closed through today, with an announcement expected today when they will reopen.
The Federal Aviation Administration grounded all commercial flights until at least noon today. Most cultural and sporting events, including major-league baseball games, were canceled or postponed.
The U.S. Atlantic Fleet dispatched aircraft carriers to Washington and New York to provide air defenses for the nation's political and financial capitals. It also sent guided-missile cruisers and destroyers, capable of thwarting air attacks, and amphibious ships, equipped with medical facilities, to New York. Officials tightened the borders with Mexico and Canada.
The city of New York - its mayoral primary promptly canceled - was largely sealed off, with tunnels and bridges into Manhattan closed and public transit, including Amtrak and Greyhound service, frozen for most of the day. Bush declared a major disaster there.
Bracing for the worst, federal health officials activated for the first time a full-blown nationwide medical emergency disaster plan aimed at identifying dead victims and caring for survivors.
Officials in New York said that 265 firefighters had been killed, and that 78 police officers were missing.
Tommy G. Thompson, secretary of the U.S. Department of Health and Human Services, said emergency teams of medical and mortuary workers, totaling more than 300 people, were being dispatched to New York and Washington to assist local workers. Emergency medical supplies were also being shipped to New York.
Eighty federal medical-disaster teams throughout the country also were ready to help, as were thousands of health workers in the private sector, Thompson said. He said his department was working with the Federal Emergency Management Agency and local health officials to assess medical needs and would provide additional personnel as needed.
Bush was taking part in a reading lesson in an elementary school in Sarasota, Fla., yesterday morning when an adviser whispered the news into his ear. He called the terrorists cowards.
"Freedom itself was attacked this morning, and I assure you freedom will be defended," Bush said.
His wife, Laura, and their 19-year-old twin daughters were moved to secure locations, as were Vice President Cheney, House Speaker J. Dennis Hastert (R., Ill.) - third in line to the presidency - and other top government officials, including members of the Supreme Court.
"We're at war. We're absolutely at war. This is 21st-century war," Rep. Curt Weldon (R., Pa.) said from Washington.
In Philadelphia, schools closed in the morning, and the city was put on emergency status shortly before 10 a.m. All buildings in Independence National Historical Park were closed, parking was banned in the blocks surrounding the Liberty Bell and Independence Hall, and federal buildings and business towers were evacuated. The state Capitol in Harrisburg also was evacuated, but the Statehouse and state offices in Trenton remained open.
When the New York towers crumpled, permanently altering the famed city skyline, clouds of dust and ash blew hundreds of feet into the air in a scene that many witnesses likened to an exploding volcano.
The devastation was visible from space.
"As we went over Maine, we could see New York City and the smoke from the fires," Frank Culbertson, commander of the International Space Station, said on NASA television from the station.
Those who were in and around the towers described a horrific scene.
"The smoke was completely engulfing me," said Peter Fink, 41, a lawyer from Long Island who was just outside. "It was so thick, so dark, so black, you literally couldn't see as far as the tip of your nose. It was like someone had duct-taped your eyes shut. . . . I was worried about smoke inhalation, about dying of smoke inhalation. I figured I didn't have too long, 40 seconds maybe."
"You could see people jumping out of the windows . . . jumping out from the highest floors," added Megan Cummins, 23, a Wall Street trader. "The debris was falling, too, and you could see the debris sort of floating. But the people weren't floating."
A physician at St. Vincent's Medical Center in Greenwich Village, the main triage center for the disaster, said last night that 327 victims - including 57 police and firefighters - had been brought in, and that three had died. Sixty-two were in critical condition.
Makeshift field hospitals and triage centers were also established in Lower Manhattan, at a sports complex on the city's Lower West Side, and in New Jersey.
Giuliani declared a state of emergency in the city, closing off the area from 14th Street south to all but residents and emergency and rescue personnel. He said violators would be arrested and prosecuted.
All four of the hijacked planes had taken off from East Coast airports - two from Boston, one from Newark, and one from Dulles, outside Washington - about 8 a.m., all heavy with fuel for intended trips to California. Spokesmen at American and United, the country's biggest carriers, confirmed that each had lost two planes.
The first jet, American's Flight 11 from Boston to Los Angeles, crashed into the north tower of the World Trade Center about 8:45 a.m., starting a fire. The second tower was promptly ordered evacuated. But 18 minutes later, while news programs were broadcasting live from the scene, United Flight 175 from Boston to Los Angeles appeared in the sky, slamming into the second tower in a crash of fire and filling the streets with smoke and ash.
"At first, everyone thought the first one was a bomb," said Dean Stamos, who owns a financial company on Wall Street. "So we were sort of watching from the office window, and we could see this other plane come in, right-side up. The second plane, it sort of twisted so it was flying on one side, and then it crashed right in to the other tower.
"There were probably thousands of us. We all went to the South Street Seaport, and we were just standing there. These planes were flying by overhead, and everyone was just staring up because we didn't know what they were. Everybody was sort of running all sorts of ways, trying to get away, but they didn't even know what they were trying to get away from."
Subsequent explosions collapsed each tower within 90 minutes. Later in the day, a 47-story building in the complex, long evacuated, also collapsed.
Shortly after the planes crashed into the towers, American Flight 77 from Dulles to Los Angeles slammed into a side of the Pentagon.
Near the Pentagon, Michael Walter, a television correspondent for USA Today Live, was stuck in traffic. He got out of his car to see what the problem was, he said, and looked up to see a plane perhaps only 20 feet over his head. "When I saw it, I said, 'Oh my God! Oh my God! Oh my God! I can't believe this. I was going into a state of shock," he said.
Shortly after, he heard a small explosion, which Pentagon officials said were propane tanks exploding near the building.
"You felt more than you heard," said Navy Cdr. Tom Rawson, a missile analyst. "First a shake, then a bang, then a boom, boom, and you knew it wasn't normal."
As rescue crews struggled to free people and firefighters battles the spreading flames, a section of the Pentagon, built during World War II, collapsed about 10:10 a.m.
Virtually all federal agencies in Washington were evacuated by midmorning.
About 10 a.m., just before the last plane went down in Pennsylvania, an emergency dispatcher received a cell-phone call from a man who said he was a passenger locked in a bathroom aboard United Flight 93 from Newark to San Francisco. The man repeatedly said the call was not a hoax, said dispatch supervisor Glenn Cramer in neighboring Westmoreland County.
"We are being hijacked! We are being hijacked!" the man said, according to a transcript of the call.
The man told dispatchers that the plane "was going down. He heard some sort of explosion and saw white smoke coming from the plane and we lost contact with him," Cramer said.
Others who called from doomed planes reported that they were calling on hijackers' orders, and said that cabins had been taken over by attackers with knives and sharp instruments and that flight attendants had been stabbed.
One passenger on the flight that struck the Pentagon was Barbara Olson, a frequent CNN commentator and the wife of Bush's solicitor general, Theodore Olson.
Lauren Grandcolas called her husband, Jack, from one of the United jets, telling him: "We have been hijacked. They are being kind. I love you," the Washington Post reported.
In New York, where the twin towers held equipment and antennas that transmit millions of calls daily, phone lines became jammed as news of the attack filtered out, making it all but impossible for people to determine if friends, workers and family members were safe.
In the West Bank city of Nablus yesterday, thousands of Palestinians poured into the streets to celebrate, chanting "God is great" and distributing candy to passers-by, even as their leader, Yasir Arafat, told reporters in Gaza that he abhorred the catastrophe.
"We completely condemn this. . . . We were completely shocked. It's unbelievable, unbelievable, unbelievable," he said.
"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. "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."
The Taliban's foreign minister, Wakil Ahmed Muttawakil, said in Kabul: "We have tried our best in the past and we are willing in the future to assure the United States in any kind of way we can that Osama is not involved in these kinds of activities."
However, terrorism expert Harvey Kushner of Long Island University said he believed bin Laden was the only person "who could pull this off."
"When you think of the coordination this took, it's historic," he said. "When you think of the measures that will have to be put into place to ratchet up security in the United States, it's monumental."
Ralph Vigoda's e-mail address is email@example.com.
Contacting the FBI
Attorney General John Ashcroft asked that anyone with information about yesterday's terrorist attacks contact the FBI via a Web site - www.ifccfbi.gov - or call 1-866-483-5137.