MORE ON THIS STORY VIDEO • Complete Video Coverage of the Attacks IN DETAIL • Terror Strikes America RELATED STORIES • U.S. Aviation Shuts Down • Descriptions of Boeing 767, 757

Government sources told ABCNEWS that during the flight, a passenger called 911 from a cell phone to report that the plane had been hijacked.

According to The Associated Press, the man told officials, "We are being hijacked, we are being hijacked!"

Emergency dispatcher Glenn Cramer said the man on board reported 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."

Information from the company Flight Explorer, which tracks the path of planes using Federal Aviation Administration data, showed the plane taking a hard turn south near Cleveland — meaning the hijacking presumably took place at that point — and then heading in the direction of Washington.

ABCNEWS has learned that shortly before changing directions, someone in the cockpit radioed in and asked the FAA for a new flight plan, with a final destination of Washington.

An FBI official said the crash "appears" to be an act of terrorism, but Pentagon officials firmly denied to ABCNEWS rumors that the U.S. military shot down the aircraft to prevent it from being crashed into Camp David, the presidential retreat in Maryland, or another government facility.

Adm. Craig Quigley told reporters the cause of the accident "was not engagement by a U.S. fighter aircraft."

Terrorist hijackings are believed to be responsible for the plane crashes this morning that demolished the World Trade Center in New York and caused part of the Pentagon in Washington to collapse.

Eyewitness Reports

One eyewitness to the Pennsylvania crash, Linda Shepley, told television station KDKA in Pittsburgh that she heard a loud bang and saw the plane bank to the side before crashing.

"It shook the whole station," said Bruce Grine, owner of an auto service center in Shanksville, about 3 miles from the crash site. "Everybody ran outside, and by that time the fire whistle was blowing."

"I heard the engine gun two different times and then I heard a loud bang and the windows of the houses all around rattled," area resident Michael R. Merringer told The Associated Press. "Everything was on fire and there was trees knocked down and there was a big hole in the ground."

The crash left a crater in a field surrounded by woods, below a hilltop strip mine. Investigators believe the plane crashed and disintegrated, spewing debris.

"There's nothing in the ground you can see," Captain Frank Monaco of the Pennsylvania State Police told reporters. Monaco said of the crash site, "It just looks like tiny pieces of debris."

According to United, there were 38 passengers and a crew of seven aboard the Boeing 757.

Working With the FBI

Another United plane, Flight 175 — a Boeing 767 headed from Boston to Los Angeles — crashed into the South Tower of New York's World Trade Center soon after 9 a.m.

Additionally, two American Airlines planes crashed this morning. American Airlines Flight 11, from Boston to Los Angeles, crashed into the North Tower of the World Trade Center just before 9 a.m. ET, and American Airlines Flight 77, heading from Dulles Airport near Washington to Los Angeles, crashed into the Pentagon.

United CEO James Goodwin said: "The thoughts of everyone at United are with the passengers and crew of these flights. Our prayers are also with everyone on the ground who may have been involved." Goodwin added: "United is working with all the relevant authorities, including the FBI, to obtain further information on these flights."

For the first time ever, the Federal Aviation Administration has ordered all to be flights canceled nationwide.

"This was a measure we took because we thought it was prudent," FAA spokeswoman Laura Brown told reporters.

ABCNEWS' Lisa Stark and Dan Harris contributed to this report.