Sept. 11, 2001  -- In the worst terrorist attack ever on U.S. soil, hijackers crashed passenger planes into the World Trade Center in New York, toppling the 110-story twin towers, killing all aboard the jets and an unknown number on the ground.

Another passenger plane crashed into the Pentagon outside Washington, leaving a gaping gash in the nation's hub of military power, killing all on board the aircraft and more on the ground. Within the hour, a fourth airliner went down near Pittsburgh, killing everyone aboard.

There were no immediate details available on casualties, but thousands of people work in the affected buildings — 50,000 in the World Trade Center alone. Hundreds of people were in local hospitals. New York Mayor Rudolph Giuliani said hundreds of firefighters were feared dead and the death count could "go into the thousands."

Giuliani also said he believed some victims were still alive underneath the rubble but firefighters were unable to reach them because of the smoke, the heat of the rubble and the amount of debris.

With a black smoke billowing over the area where the World Trade Center towers once stood, city officials also said the dead were buried under several feet of soot and debris and that machines would be need to lift some of the rubble to help firefighters with the body recovery.

"It's tough. … we should hug one another and give thanks that we're all here," Giuliani said. "Tomorrow is another day. New York is still here, and we will rebuild and be stronger than ever."

There were reports that officials at Port Authority of New York and New Jersey had stopped a van at the George Washington Bridge that may have contained explosives, but Port Authority and Fort Lee, N.J., police said those claims were untrue.

No Longer Security as Usual

Meanwhile at the Pentagon, the smell of smoke, jet fuel and gas was still rising as some of the dead lay outside the damaged building. The entire area was deserted of civilian and military employees, but dozens of emergency personnel were stationed outside the building doing triage on the wounded and searching for bodies still underneath the debris.

As Pentagon officials identified the dead, they began contacting next of kin. The chief of the Arlington, Va., Fire Department estimated that were between 100 and 800 people dead at Pentagon.

Washington, D.C., Police Chief Charles Ramsey said security in the area would be no longer "business as usual" and that officials were still assessing whether roads will continue to be closed Wednesday. Ramsey's department also recommended that next month's meeting between the World Bank and the International Monetary Fund in Washington be canceled or postponed.

In an address to the nation tonight, President Bush vowed that the terrorists would be brought to justice.

"The functions of our government will continue without interruption," Bush said from the White House. "Our financial institutions remain strong. The American economy will be open for business as well. We have a full resolution to find those responsible for this evil act and bring them to justice. We will make no distinction between terrorists who committed these acts and those who harbor them."

President Bush was meeting with his National Security Council staff tonight and was expected to remain at White House until Wednesday morning.

FBI Investigation Under Way

Attorney General John Ashcroft said thousands of FBI agents from around the country are being assigned to investigate the attacks and will be focusing on New York, Washington, Pittsburgh, Boston and Newark, N.J. Ashcroft also pledged that those responsible would be brought to justice.

"The determination of the terrorists will not deter the determination of the American people," said Ashcroft. "We will find the people responsible for this terrible act and justice will be done."

According to the FBI, the planes involved in the World Trade Center crashes were American Airlines Flight 11, a Boeing 767 en route from Boston to Los Angeles carrying 81 passengers, nine flight attendants and two pilots, and United Airlines Flight 175, a Boeing 767 also headed from Boston to Los Angeles. Officials said the plane was carrying 56 passengers, two pilots and seven flight attendants.

FBI officials have begun interviewing airline officials at Dulles and Boston's Logan airports in an attempt to get a sense of who was on the airplanes. The FBI has also set up a Web site at www.fccfbi.gov for those who have any information about the terrorist attacks.

Tonight, the FBI issued several search warrants on people who live within the United States based on preliminary information on how two of the planes were hijacked. They said a flight attendant called authorities from the airborne American Airlines Flight 11, reporting that two flight attendants had been stabbed and that intruders had broken into the cockpit. The Christian Science Monitor reported that air traffic controllers could hear the hijackers of Flight 11 instructing the pilots in English.

On the United flight that crashed outside Pittsburgh, a passenger called authorities from the plane on his cell phone, saying the plane had been hijacked. A senior FBI official also told ABCNEWS that investigators have received reports that other passengers were able to use their cell phones to call 911 or relatives and describe horrific scenes in which terrorists were slashing people's throats.

In an Instant, Skyline Changes

The chaos began at about 8:50 a.m. ET when a hijacked American Airlines passenger plane smashed into the 110-story One World Trade Center, the northern tower. Then, at about 9 a.m., another jet crashed into the southern tower, Two World Trade Center.

At about 10 a.m., the southern tower collapsed, enveloping lower Manhattan in a cloud of dust, ash and debris. A half-hour later, the northern tower also fell in on itself and the New York City skyline was indelibly changed.

"Lots of smoke and then the next thing I heard was an explosion in the building from the top, the south building just crumbled, just completely went down, I saw it," said witness Joan Fleischer. "It's hard to see all the pieces, but you could see it tipping over and just crashing to the ground."

Shortly after the crashes at the World Trade Center, at about 9:40 a.m., FBI officials say American Airlines Flight 77, a Boeing 757 en route from Washington Dulles Airport to Los Angeles crashed into the Pentagon. The plane was carrying 58 passengers, four flight attendants and two pilots.

Then, at about 10:40 a.m., reports came that United Airlines Flight 93, a Boeing 757, en route from Newark to San Francisco, crashed in western Pennsylvania, about 80 miles southeast of Pittsburgh. The plane was carrying 38 passengers, two pilots and five flight attendants.

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.

Evacuations Occur Nationwide

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.

Bin Laden 'High on List of Suspects'

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.

There was no indication that there was any state sponsorship of today's attacks, the officials said.

Bin Laden warned three weeks ago that his followers would carry out an "unprecedented attack" on the United States, an Arab journalist told Reuters news agency.

Along with several other world leaders, Palestinian leader Yasser Arafat condemned the aircraft attacks. Arafat spoke shortly after an anonymous caller told Abu Dhabi television that a radical Palestinian group was responsible for the attacks. The group later denied any involvement.

"I send my condolences, the condolences of the Palestinian people to American President Bush and his government and to the American people for this terrible act," Arafat told reporters in Gaza. "We completely condemn this serious operation … We were completely shocked. It's unbelievable, unbelievable, unbelievable."

Standing Together

House and Senate leaders stood side by side in a show of unity this evening and vowed to stand together behind the nation and Bush. They said that the acts of terror would not stop Congress from convening Wednesday.

In Kabul, Afghanistan's capital, early Wednesday morning but this evening in Washington, video from a Middle Eastern news agency showed flames, explosions and tracer fire. Bin Laden, considered to be one of the world's most dangerous terrorists, is said to be based in Afghanistan.

White House spokesman Ari Fleischer said the attacks in Kabul were not ordered by the United States. Pentagon spokesman Craig Quigley said: "It isn't us. I don't know who's doing it."

'I Don't Believe This'

In lower Manhattan, as police, fire and emergency personnel tried to cope with the mayhem, witnesses were in shock.

"I'm sitting down and I'm crying and I couldn't believe that something like this could actually happen," said Tony Bristow, who was working on a nearby pier when the planes smashed in the towers. "Then about 10 minutes later the whole building just started to collapse and now two seconds ago both of them collapsed and now there's no more World Trade Center. It's — this is ridiculous. I don't believe this."

The tops of the twin towers were obscured by the smoke, and thousands of pieces of what appeared to be office paper came drifting over Brooklyn, about 3 miles from the tower.

Before they collapsed, large holes were visible in sides of the landmark twin towers, which were struck by terrorist bombers in February 1993, killing six people and injuring more than 1,000 in attack that at that time shook America.

ABCNEWS' Geraldine Sealey, Dean Schabner, Michael McAuliff, Bryan Robinson, Pierre Thomas, Barbara Starr and Corey Goldman contributed to this report.