SHANKSVILLE, Pa. -- Two United Airlines jetliners crashed Tuesday morning, one in western Pennsylvania and the second at a location the airline did not immediately disclose. A total of 110 people were aboard the two planes, the airline said.

Today's air tragedies

American Airlines Flight 11: A Boeing 767 en route from Boston to Los Angeles. The plane was carrying 81 passengers, nine flight attendants and two pilots. The location of the crash has not been determined. American Airlines Flight 77: A Boeing 757 en route from Dulles Airport near Washington to Los Angeles. The plane was carrying 58 passengers, four flight attendants and two pilots. The location of the crash has not been determined. United Airlines Flight 93: A Boeing 757, crashed southeast of Pittsburgh while en route from Newark, N.J. to San Francisco. The plane was carrying 38 passengers, two pilots and five flight attendants. United Airlines Flight 175: A Boeing 767. The flight was bound from Boston to Los Angeles. It was carrying 56 passengers, two pilots and seven flight attendants. The airline would not say where the plane crashed.

One plane, United Flight 93, crashed north of the Somerset County airport, a small airport about 80 miles southeast of Pittsburgh.

The Pennsylvania crash followed the crash of two planes into the World Trade Center in New York City. American Airlines initially said its planes crashed into the twin towers but later said that was unconfirmed.

"It shook the whole station," said Bruce Grine, owner of Grine's Service Center in Shanksville, about 2½ miles from the crash. "Everybody ran outside, and by that time the fire whistle was blowing."

United said that flight, a Boeing 757, left Newark, N.J., at 8:01 a.m., headed for San Francisco with 38 passengers, two pilots and five flight attendants.

A second plane, United Flight 175, a Boeing 767, also crashed, the airline said, but it did not give a location. That plane left Boston at 7:58 a.m., bound for Los Angeles with 56 passengers, two pilots and seven flight attendants, the airline said.

United's pilots union said Flight 175 crashed into the Trade Center. But the airline had no immediate comment.

Because of the attacks in New York, the Federal Aviation Administration had ordered all departing flights canceled nationwide, and any planes already in the air were to land at the nearest airport. The Pennsylvania crash came after the order was issued.

According to Somerset County dispatchers, Flight 93 crashed about 10 a.m. about 8 miles east of Jennerstown, WPXI-TV in Pittsburgh reported.

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

"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."

The couple rushed home and drove near the scene.

"Everything was on fire and there was trees knocked down and there was a big hole in the ground," he said.

Earlier Tuesday, two planes crashed into the World Trade Center, causing the twin 110-story towers to eventually collapse. A plane also hit the Pentagon in Washington.

In Chicago, United CEO James Goodwin said the airline is working with authorities, including the FBI. United said it was sending a team to Pennsylvania 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.

At Boston's Logan Airport, Joseph Lawless, director of public safety for the Massachusetts Port Authority, said a family assistance center had been set up and families were arriving at the airport.

The three passenger terminals at Newark International Airport were evacuated. At 11:30 a.m., several hundred people were still clustered at the Terminal A baggage carousel, while shotgun-toting officers patrolled. Ticket counters were deserted.