MORE ON THIS STORY FULL COVERAGE • America Under Attack VIDEO • Hijacked Plane Hits First Tower • Hijacked Plane Hits Second Tower • Complete Video Coverage of the Attacks IN DETAIL • Terror Strikes America COMMUNITY • Your Reaction to the Terrorist Attacks

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Investigators believe at least some of the hijackers were U.S.-trained pilots who entered the country through Canada. State Department officials said they have intelligence information connecting the attacks to indicted terrorist Osama bin Laden.

Most, if not all, of the hijackers were Egyptian or Saudi nationals, sources said.

White House spokesman Ari Fleischer and Secretary of State Colin Powell said there was reason to believe the plane that hit the Pentagon was intended to strike the White House, and that Air Force One was also a target. They refused to elaborate on the "credible information" that led to that belief.

A senior administration official told ABCNEWS that the plane's flight pattern suggested the White House was targeted. The plane circled over the White House and then, for unknown reasons, headed for the Pentagon. There was speculation Tuesday that the fourth plane that crashed in Pennsylvania also targeted Washington.

Getting Worse in New York

In New York, conditions deteriorated late this afternoon when another building at the World Trade Center, Building 5, partially collapsed at One Liberty Plaza. Engineers warned and evacuated rescue workers before the building began to sag. Firefighters, who work separately from the rescue crews, were the only ones allowed in the area amid concerns that there were gas leaks. In addition, rescue workers were evacuated shortly before the remaining seven stories of the South Tower of the World Trade Center fell into the rubble.

Engineers were also concerned about the condition of four other buildings, including the Millenium Hilton Hotel and the 54-story Merrill Lynch Building, which they described as "shakey" to ABCNEWS. Some residents had to be evacuated from their apartments because health officials were concerned about the bad air quality that resulted from the building debris. The impact of Tuesday's attack caused damage to buildings within several blocks of the World Trade Center towers, and rescue crews are concerned that additional collapses could hinder rescue and recovery attempts. On Tuesday, hours after the towers collapsed, Building 7 in the World Trade Center collapsed.

Meanwhile, the United States today received a pledge of international support from its NATO allies, who declared the attacks in the United States can be considered an attack on the whole alliance if it turns out they were directed from abroad.

"An attack on one is an attack on all," NATO Secretary-General Lord Robertson said after the 19 NATO ambassadors decided to invoke Article 5 of the NATO charter for the first time in the alliance's history.

The decision means the United States can count on military support from its NATO partners, if necessary.

Possibly Thousands of Bodies in the Rubble

One day after the deadliest terrorist attack in U.S. history, the extent of the carnage was still not known. Thousands were believed trapped in the rubble of the former site of the World Trade Center, which was leveled when two hijacked airplanes smashed into its famous twin towers Tuesday morning.

At the Pentagon, where a third hijacked passenger jet slammed into a section of the nation's military headquarters, officials estimated the number of people dead or missing there at about 200.

As for the four hijacked passenger planes — another jet was downed outside Pittsburgh on Tuesday — the death toll was 266 passengers and crew. All the flights were bound for California. Two originated in Boston, one from Washington Dulles International Airport in Virginia, and one from Newark, N.J.

At the twin towers wreckage site, officials said there was still a chance for miracles, and there have been a few. One police officer and six firefighters were pulled alive from the wreckage today. Two other police officers were rescued from the rubble during the night.

At trauma centers in Manhattan, legions of medical personnel — many of them volunteers — waited to treat victims, but ominously, fewer victims than expected arrived for care. News reports were filled with anguished people searching for missing friends and relatives.

Some 50,000 people worked in the World Trade Center, but it was unclear how many were inside at the time of the attack, or how many made it out before the buildings fell. About 24,000 people work at the Pentagon.

At least 202 firefighters were still missing and 57 police officers had not been accounted for in New York City.

"Everyone I know is dead," a rescue worker in New York said, referring to fellow rescuers who were buried in the ruins when the towers came crashing down.

A Massive Manhunt

While the frenzied rescue efforts continue, the federal government is conducting a worldwide investigation into what President Bush called "acts of war."

The probe is also reaching into cyberspace. Two of the nation's major Internet service providers, America Online and Earthlink, have been served with "surveillance orders" by the FBI, which is apparently looking for Internet and e-mail traffic connected to the attacks. Investigators have found clues in Boston, Maine and Florida that helped identify the hijackers, and pointed to possible accomplices.

In Boston, where two hijackings originated, the FBI identified five Arab men as suspects, at least two of whom are believed to have flown into Boston from Portland, Maine, early Tuesday and were booked on the flights to Los Angeles.

Officials said investigators believe the hijackers used cash and credit cards to pay for tickets and hotel rooms.

The men may have entered the United States from Canada, and are believed to have "cased" Logan International Airport, making several trips back and forth between Portland and Boston.

FBI Director Robert Mueller said investigators are trying to identify and find relatives or anyone associated with the hijackers who may have had some involvement in the terrorist attacks. He stressed that no one has been arrested in the attacks.

Some people have been detained for questioning.

One man and two women were taken into custody today when heavily armed FBI teams searched a room in the Westin Hotel at Boston's Copley Plaza for information related to the attacks. Investigators say the three are being held on immigration-related charges. Police also detained one man after a Washington-bound Amtrak train originating from Boston was stopped in Providence, R.I., but officials at the Department of Justice said the train stop had nothing to do with the FBI's investigation and that Boston police decided to make the stop.

The Boston Globe reported that a copy of the Koran, instructions on how to fly a commercial airplane and a fuel consumption calculator were found in a pair of bags meant for one of the hijacked flights that left from Logan.

Two planes crashed into the World Trade Center. (ABCNEWS.com/ Magellan Geographix)

At about 9 p.m. Tuesday, FBI agents and Massachusetts State Police towed a rented, late-model, white Mitsubishi Mirage with a Virginia license plate to an FBI garage. Arabic-language flight training manuals reportedly were found inside the car.

Investigators were led to the car by another airline passenger who got into a dispute with two men in the car over a parking space. When he heard about the planes that crashed into the World Trade Center, he called authorities in Boston.

In Florida, authorities traced a 1989 Pontiac Grand Prix registered to a trained pilot and Egyptian national, Mohamed Atta, 33, to a Venice, Fla., address. Sources identify Atta as one of the hijackers.

Agents who reported to the address learned that Atta did not live there, but had stayed at the home last year while getting flight instruction. Agents also went to a Coral Springs, Fla., address listed on Atta's driver's license, sources said.

ABCNEWS sources identify another hijacker as Satan Suqami, a Saudi national on American Airlines Flight 11, whose passport was recovered in the rubble.

The FBI is also investigating possible connections Atta and another man only identified as Alchennen had in Hamburg, Germany. Acting on a tip from the FBI, a German SWAT team searched a residence where Atta and Alchennen allegedly once lived and found it empty. According to the investigators, neighbors said the home had been empty since February. Investigators attempted to collect fingerprints and other potential evidence.

A Connection?

Rudy Dekkers, CEO of Huffman Aviation in Venice, Fla., told ABCNEWS today that two men, one from Afghanistan and one from another country, had trained at his flight school from July 2000 to November, taking instruction on small planes. They got licenses and left for a southeastern Florida facility where they would be able to train on jets, he said.

A senior law enforcement official said two of the men suspected in the hijackings was trained at Huffman for at least a year. One was certified by the FAA. Huffman Aviation, Justice Department officials said, is just one of a number of different schools in several states that investigators are looking at.

The FBI asked Dekkers to give them documents about the two individuals and their training. He said he gave investigators all the documentation plus information on other students. "We fly hundreds of students from all over the world," he said.

In addition, a restaurant manager in Hollywood, Fla. told The Associated Press that FBI agents showed him a picture of two men suspected in the hijacking and that he remembered seeing them in his bar last Friday. Tony Amos said one of the men said his name was Mohamed and that he was a pilot for American Airlines.

In Maine, authorities impounded a blue Nissan Altima found at the Portland International Jetport that they believe provided the transportation for five suspected terrorists. Investigators suspect the car was driven to Bangor, Maine, where three of the suspected terrorists allegedly boarded a flight to Boston. Then, the other two suspects drove the car back to Portland and boarded a separate flight to Boston.The car is at the Maine State Crime Lab in Augusta.

Beginning to Understand

Investigators are beginning to understand how hijackers may have overpowered the flight crews and passengers to gain control of the doomed airplanes in what appears to have been a highly coordinated operation.

Once in the air, the hijackers of the Boston flights used box cutters and other knife-like weapons made of plastic handles embedded with razor blades to kill flight attendants — and lure a pilot from the cockpit, after which they were able to seize control of the plane, sources said.

One of the flight attendants on American Airlines Flight 11 was able to call authorities and give some details of the hijacking before the jet crashed into the World Trade Center. Along with the knife-like weapons, the flight attendant said the hijackers used cans of mace to immobilize business- and first-class passengers.

While searches are under way for survivors and suspects, the nation is struggling to resume some semblance of normalcy.

The Federal Aviation Administration will allow a limited reopening of the nation's commercial airspace system in order to allow flights that were diverted Tuesday to continue to their original destinations, Transportation Secretary Norman Mineta said today.

But the unprecedented nationwide ground-stop order for airports will continue at least through today while new security measures are being completed, he said. Those measures include a thorough search and security check of all airplanes and airports before passengers are allowed to board aircraft, and the end of curbside and off-airport check-in. Boarding areas will be reserved for passengers only, officials said, and only ticketed passengers will be allowed past airport screeners.

However, the more measures will be taken to ensure flight safety. Sources tell ABCNEWS that the federal government Thursday will announce plans to significantly increase uniformed police patrols at airports. A "check point" program that will place additional U.S. marshals, border patrol and customs agents at many airports will be implemented as soon as possible. Vehicles near airports also will be monitored more closely.

Meanwhile, Major League Baseball, which cancelled schedule games Tuesday, called off games for the second straight day. The National Football League is also considering cancelling games scheduled this weekend and Monday. And the New York Stock Exchange, the Nasdaq Stock Market and the American Stock Exchange will remain closed at least until Friday.

Was It Bin Laden?

A plane crashed into the Pentagon. (ABCNEWS.com/ Magellan Geographix)

Bin Laden has been indicted as the mastermind behind the 1998 attacks on U.S. embassies in Kenya and Tanzania, and quite possibly the deadly attack on the USS Cole last year. There is a $5 million price on his head.

He has denied responsibility for Tuesday's attacks, and a Pakistan-based newspaper quoted bin Laden as blaming "some American group."

According to Reuters, the Urdu-language Khabrain has a reputation for sensational reporting, and there was no independent confirmation of the claim. However, Afghanistan's ruling Taliban also denied that bin Laden was involved.

The FBI has set up a Web site (http://www.ifccfbi.gov) for those with information about the attacks.