Four American Airlines and United Airlines passenger jets were hijacked and used in a coordinated terrorist attack on American targets Tuesday, with two planes crashing into, and eventually collapsing, the twin World Trade Center towers in New York City and a third destroying part of the Pentagon outside Washington, D.C. 

The fourth plane crashed into a rural area in Pennsylvania, about 80 miles southeast of Pittsburgh. It was not known what caused that aircraft to crash so far from any obvious targets, leading to speculation that it may have been shot down by the military. 

A Virginia congressman, Rep. James Moran, said the intended target of the Pennsylvania plane was apparently Camp David, the presidential retreat in Maryland, 85 miles away. 

All four flights were transcontinental flights, with three heading to Los Angeles and one to San Francisco. They may have been selected because their fuel tanks would have been nearly full upon impact. 

The Federal Aviation Administration issued a nationwide "ground stop" at about 9:25 a.m. EDT Tuesday morning, ordering all departing flights canceled nationwide and any planes already in the air to land at the nearest airport. Flights heading to the United States from overseas were redirected to Canada. 

Unconfirmed reports initially said that as many as eight airliners may have been hijacked, but there were no more crashes reported after 10 a.m. EDT. as the skies cleared. 

"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 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 [are] on that list. ... I would name at the top of the list Osama bin Laden." 

The nearly simultaneous attacks pointed to a meticulously planned strike that may well have employed trained pilots, other experts on terrorism said. 

"No pilot, even with a gun to his head, is going to fly into the world towers," said Gene Poteat, president of the Association of Former Intelligence Officers. 

"They [the presumed terrorists] flew the planes themselves," he speculated. 

American Airlines Flight 11, on its way from Boston to Los Angeles, was hijacked Tuesday morning shortly after takeoff from Logan International Airport. The airline said it was a Boeing 767-200, carrying 81 passengers, nine flight attendants and two pilots. 

American stated that Flight 11 was the plane that struck Two World Trade Center, the northernmost of the twin towers on the southern tip of Manhattan, crashing into it at about 8:50 a.m. EDT. 

About ten minutes later, another commercial jet, apparently United Airlines Flight 175, a Boeing 767 bound from Boston to Los Angeles, approached from the southwest and crashed directly into of Two World Trade Center, the south tower. 

United Airlines could only say that Flight 175 lost radio contact between Newark, N.J., and Philadelphia. It carried 56 passengers, two pilots and seven flight attendants. 

A second American Airlines plane, Flight 77, en route from Washington, D.C., to Los Angeles, was thought to be the craft that crashed into the Pentagon. It was a Boeing 757 carrying 58 passengers, four flight attendants and two pilots. 

Fox News commentator Barbara Olson, wife of U.S. Solicitor General Theodore Olson, was among those aboard Flight 77. Sources tell Fox News that Ms. Olson called her husband from her cell phone twice during the flight, telling him that the hijackers were using knife-like instruments to control the passengers in crew. 

Fox News has also learned that American Airlines was fined $99,000 by the Federal Aviation Administration two months ago for failing to apply appropriate security measures on six flights, two of which originated in Boston. 

Undercover FAA special agents found that American security screening transported unaccompanied baggage, failed to properly check passenger identification and did not ask appropriate security questions of passengers. 

United Airlines has no related security breaches, but was fined recently for shipping inappropriate flammable materials. 

Both towers of the World Trade Center collapsed within hours as their internal structures failed, apparently from the heat of the fires. 

Military jets were spotted circling New York City at about 11 a.m. EDT. 

Two World Trade Center, with a television broadcast tower adding several hundred feet to its height, was briefly the world's tallest building when it was erected in 1972. It was surpassed by Chicago's Sears Tower in 1975, which in turn was topped by the Petronas Towers in Kuala Lumpur, Malaysia, in 1998. 

Several New York television and radio stations, including WABC-TV and WNBC-TV, effectively went off the air when Two World Trade Center collapsed. 

Four hundred miles to the southwest, American Flight 77 slammed into the Pentagon at about 9:45 a.m., within an hour of the attacks in New York City. 

Glenn Flood, a Pentagon spokesman, said there were "extensive casualties and an unknown number of fatalities. "We don't know the extent of the injuries," he said. 

"The leadership of the Defense Department is OK. The secretary [Secretary of Defense Donald H. Rumsfeld] is OK," Flood told reporters. 

"The whole building shook" with the impact, said Terry Yonkers, an Air Force civilian employee at work inside the Pentagon at the time of the attack. "There was screaming and pandemonium," he said, but the evacuation ordered shortly afterward was carried out smoothly. 

"I saw a big jet flying close to the building coming at full speed. There was a big noise when it hit the building," said Oscar Martinez, who witnessed the attack. 

The departments of Justice, State, Treasury and Defense and the Central Intelligence Agency were evacuated — an estimated 20,000 at the Pentagon alone. Agents with automatic weapons patrolled the White House grounds. 

About half an hour later, part of the Pentagon, one of the world's largest structures and the headquarters of the U.S. military, collapsed as well. 

Fox News was told that the crash at the Pentagon could have been a lot worse, as the wedge of the Pentagon that was hit recently underwent renovations and was largely "unoccupied" at the time of the impact. 

However, there are reports of "dozens" of fatalities, perhaps from adjacent offices or from passengers on board the aircraft. 

United Airlines Flight 93, a 757 en route from Newark, N.J., to San Francisco, crashed about 10 a.m. about 8 miles east of Jennerstown, Pa. 

It was carrying 38 passengers, two pilots and five flight attendants. 

The Somerset County airport, where Flight 93 plane may have trying to land, is a small, rural facility that does not handle such aircraft. 

An emergency dispatcher in neighboring Westmoreland County received a cell phone call at 9:58 a.m. from a man who said he was a passenger locked in the bathroom on that flight, said dispatch supervisor Glenn Cramer. 

The man repeatedly told officials the call was not a hoax. 

"We are being hijacked, we are being hijacked!" Cramer quoted the man from a transcript of the call. 

The man told dispatchers 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. 

"It shook the whole station," said Bruce Grine, owner of Grine's Service Center in Shanksville, Pa., about 2 miles from the crash. "Everybody ran outside, and by that time the fire whistle was blowing." 

Michael R. Merringer was out on a mountain bike ride with his wife, Amy, about two miles away from the crash site of Flight 93. 

"I heard the engine gun two different times and then I heard a loud bang and the windows of the houses all around rattled," Merringer said. "I looked up and I saw the smoke coming up." 

In Chicago, United CEO James Goodwin said the airline is working with authorities including the FBI. United said it was sending a team to Johnstown, Pa., to assist in the investigation and to provide assistance to family members. 

"Today's events are a tragedy and our prayers are with everyone at this time," Goodwin, said. 

Los Angeles International Airport, to which three of the four hijacked airplanes were heading, was under tight security. Police were not allowing people to unload baggage to board flights at the American Airlines terminal. 

At least 20 Los Angeles police officers were at the terminal. 

One officer Lt. Howard Whitehead said, "All we know is what we've heard from radio and television. We're not allowing any unattended vehicles near the terminals." 

Vehicle traffic was extremely light at the terminal area and the parking lot near the American terminal and the large international terminal were about only about one-third full. 

Many people with canceled flights were at the American counter seeking information. Others were carrying and pulling their bags, looking for taxis and other ways to leave after their flights were canceled. 

The Associated Press contributed to this report