The first sign of trouble aboard United Flight 93 came 15 minutes after the last of three other jetliners had crashed in New York and Washington on Tuesday.
A man who identified himself as a passenger, who said he was locked in one of the plane's restrooms, told emergency dispatchers by cell phone that the plane had been hijacked.
Eight minutes later, at 10:06 a.m., the United Airlines jet carrying 45 people crashed in a rural field in western Pennsylvania.
Local officials said there were no survivors.
There were strong indications that the crash of United Flight 93 was connected with three airborne terrorist attacks carried out Tuesday in Washington and New York. However, officials said there was no definitive evidence.
Jeff Killeen, special agent in charge of the Pittsburgh office of the FBI, said the agency was treating the crash site as a crime scene, but he did not offer any details.
"It's the totality of the circumstances of all the cases," Killeen said, referring to the New York and Washington crashes. "It would be pure folly for us to assume at this point that this was an accident."
Killeen said Tuesday evening that 20 agents were working at the scene and that an additional 30 would likely join them before the night was over.
The plane crashed two hours after it left Newark, N.J., en route to San Francisco.
Rep. James P. Moran (D-Va.) said lawmakers were told that the plane had diverted from its westbound route and had swung back toward Washington, D.C.
Moran, who said he had been briefed by the Capitol Police and other agencies, said the plane's intended target in Washington, if any, was unclear. "It was on an arc that was headed toward the capital," Moran said. "But that was also an arc in the vicinity of Camp David," the presidential retreat in Thurmont, Md.
The crash site was about 80 miles northwest of Camp David and 80 miles southeast of Pittsburgh.
Data from Flight Explorer, a Virginia-based flight-tracking service for airlines, shows Flight 93 passing north of Pittsburgh and turning around over Cleveland. The Boeing 757 then passed just south of Pittsburgh before crashing about 25 miles south of Johnstown, Pa.
At 9:58 a.m., about eight minutes before the crash, emergency dispatchers received a cell phone call from a man who said he was a passenger locked in the plane's bathroom. The man said "basically, it was a hijacking," said Edward Milliron, an emergency dispatcher in Westmoreland County, Pa. "Then we sort of concentrated on getting a flight number, where they left, where they were going, and then we lost them. Two or three minutes later, we lost them."
A second Westmoreland County dispatcher, Glenn Cramer, told reporters that the man said: "We are being hijacked, we are being hijacked!" He repeatedly said the call was not a hoax.
The plane was carrying 38 passengers, two pilots and five flight attendants, United Airlines said. The airline did not immediately release names.
The plane crashed in a field near a strip mine in rural Shanksville, Pa., which has a population of about 700.
"I heard this noise like a divebomber; you know, one of those planes they use in war. And the house was shaking," said Paula Pluta, 33, who lives less than a mile from the crash site.
Pluta said she ran out to her porch, looked up and saw the plane plummeting at an angle close to 90 degrees. "It looked like a silver bullet," she said.
The plane disappeared behind a line of maple trees, she said. Next came a tremendous roar, and flames erupted above the tree line.
"I felt it. My house was shaken by it. I thought a truck hit my house," said the Rev. Sylvia Baker of the Shanksville Assembly of God Church, who lives about two miles from the crash site. "When I saw it wasn't my house, I felt surely it had to be my neighbor's house."
The plane was largely obliterated in a crater about 30 feet by 30 feet, and about 20 feet deep. Local officials said the largest piece found was a part of the fuselage about the size of a car door.
They added that no one on the ground was hurt.
Pennsylvania Gov. Thomas J. Ridge said Tuesday evening that authorities were still trying to locate the plane's "black box," which would contain crucial flight data. He said the FBI was working with other agencies at the scene.
Joseph McKelvey, manager of the Johnstown Cambria County Municipal Airport, said the plane was spotted near Johnstown flying at about 6,000 feet. Airport officials were immediately concerned because that altitude was unusually low, he said.
McKelvey said controllers were also "alarmed" because the plane was heading toward them just after the air attacks in New York.
Then, a call came from officials at a Cleveland airport saying that there may be trouble with the flight, McKelvey said.
He said controllers in Johnstown tried to make radio contact with the plane but they did not succeed.
The Pentagon denied a rumor, which had swept through the national security community in Washington early on Tuesday, that the U.S. military had shot down the plane to prevent it from striking a target and taking additional lives.
Milliron said that although residents called 911 dispatchers to report a jet flying low over their houses, no residents said they saw a military aircraft or second plane in the area.
In a statement, United Airlines Chief Executive James E. Goodwin said that the airline was working with several government agencies, including the FBI, and that it had sent teams to the Johnstown and New York City areas to assist authorities and families.
"Our thoughts and prayers continue to be with everyone involved," Goodwin said.