SHANKSVILLE, Pa. -- A Tuesday morning plane crash that apparently killed 45 people was being investigated as an act of terrorism, according to an FBI agent.

United Airlines Flight 93 from Newark to San Francisco crashed around 10 a.m. in a wooded area about 60 miles southeast of Pittsburgh, and just north of Somerset County Airport.

A Virginia congressman said Camp David, the presidential retreat in Maryland, may have been the intended target, but later retracted the statement, saying that he never made the comment.

There were reportedly 38 passengers, two pilots and five flight attendants on board the Boeing 757. University of Pittsburgh Medical Center emergency workers who were sent to the scene told affiliate Web site ThePittsburghChannel.com that there were no signs of life.

The Newark-to-San Francisco flight reportedly diverted its route near Cleveland and ignored calls from Pittsburgh International Airport controllers. It also was reported that a request to change the plane's flight plan was called in to controllers.

Allegheny County Emergency Management Coordinator Bob Full said that Pittsburgh International Airport officials had knowledge of the plane headed for Pittsburgh and that it was taking precautionary action when the plane diverted its course. The plane actually reached the Cleveland area before turning around.

A witness said he heard two loud bangs before watching the plane take a downward turn of nearly 90 degrees. A large crater, from which none of the plane could be seen, was left in the ground.

The plane crashed into a wooded area. Many witnesses said that their homes were shaking violently as the plane flew low overhead.

A witness told WTAE-TV's Paul Van Osdol that she saw the plane overhead. It made a high-pitched, screeching sound. The plane then made a sharp, 90-degree downward turn and crashed.

Officials said that they believed that the plane took a dip and nose-dived into an abandoned strip mine.

Click here for a slideshow of the crash scene, including pictures of debris from the site.

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Another witness, Michael Merringer, said he was out on a mountain bike ride with his wife, Amy, about two miles from the crash site.

"(I) heard the engine gun two different times," he said. "(I) heard a loud bang and the windows of the houses all around rattled."

Merringer said he saw the smoke rising and he and his wire drove near the scene. "Everything was on fire and there were trees knocked down and there was a big hole in the ground," he said.

Pittsburgh's WTAE-TV reporter Michelle Wright toured the crash scene and said that a crater of about 30 to 40 feet long, 15 to 20 feet wide and 18 feet deep was created by the crash.

Officials told WTAE's Marcie Cipriani that it looked like the plane was headed south when it hit the ground. Most of the plane's debris kept traveling after the plane hit and landed in the woods past the mine. Most of the debris is small.

Crews said that they have faith that the flight data recorder, or "black box," will be found.

FBI Agent Wells Morrison said that Westmoreland County 911 received a call at 9:58 a.m. from a man who said he was on the plane. The FBI had the tape but would not discuss its contents.

The caller said he was locked in the plane's bathroom and that the flight had been hijacked, according to Glenn Cramer of Westmoreland County 911. A loud noise was then heard and the caller reported seeing white smoke, Cramer said.

Connection with the man's cellular telephone was then lost. The crash was reported soon after.

Officials from the FBI and the Federal Bureau of Alcohol, Tobacco and Firearms had sealed off the scene. It was not known whether the incident was related to terrorist plane crashes Tuesday at the World Trade Center and the Pentagon.

Call United at 800-932-8555 or visit www.united.com for information about the flight.