SHANKSVILLE, Pa. As hijackers took over their airliner in the skies over Pennsylvania, at least three passengers placed phone calls aboard United Airlines Flight 93. One was a desperate plea for help, the others were last chance calls to loved ones. Were being hijacked! one man repeatedly told dispatchers who answered 911 lines in Westmoreland County on the ground below. Another man called his mother to tell her three men had taken over the plane. He said he loved her. Then the phone went dead. The Boeing 757 slammed into a grassy field about 80 miles southeast of Pittsburgh. Rescue crews who reached the scene shortly after 10 a.m. found a deep V-shaped gouge filled with smoldering rubble. Forty-five people had been on board. Authorities say Tuesdays hijacking was part of a fourth terrorist attack, following two closely timed crashes into New Yorks World Trade Center and a third into the Pentagon. U.S. officials told The Associated Press on condition of anonymity that the Secret Service had alerted the White House that the hijackers may have been headed for Camp David, the presidential retreat in Maryland. Fearing the White House might be a target, the Secret Service diverted President Bush, who had been in Florida, to Louisiana and then Nebraska. It wasnt clear what caused the plane to go down when it did. Flight 93 left Newark at 8:01 a.m. EDT headed for San Francisco. As it approached Cleveland, radar showed the plane banked left and headed back toward southwest Pennsylvania. On board, flight attendant CeeCee Lyles grabbed her cell phone and call her husband and four sons in Fort Myers, Fla. She called him and let him know how much she loved him and the boys, said her aunt, Mareya Schneider. During the call, he heard people screaming in the background, Schneider said. In San Francisco, Alice Hoglan picked up her phone about 9:45 EDT to the hear the voice of her son, Mark Bingham, 31.He said, I want you to know I love you very much. Im calling you from the plane. Weve been taken over. There are three men that say they have a bomb, Hoglan said. The phone went dead a short time later. The caller who reached emergency dispatchers said he was inside a locked bathroom on the plane. Dispatcher Glenn Cramer said the man repeatedly said, Were being hijacked! and that his call was not a hoax. He heard some sort of explosion and saw white smoke coming from the plane and we lost contact with him, Cramer said. The man never identified himself. Tuesday night, FBI agents and forensic archeologists began picking through tiny pieces of rubble. Neither the cockpit voice recorder nor the flight data recorder had been recovered, and it was expected to be days before the victims could be identified. One of the things we need to understand is that cowardly acts of terrorism can happen at any place, at any time, Gov. Tom Ridge said. Im not sure you can ever adequately prepare for them. There were 266 people aboard the four hijacked airplanes that crashed Tuesday; officials couldnt estimate how many people died on the ground. In Pennsylvanias Richland Township on Tuesday morning, police Chief Jim Mock said air traffic control coordinators reported a large aircraft heading toward the John Murtha Johnstown Cambria County Municipal Airport. The controllers said the aircraft would not identify itself. Minutes later, the plane crashed in rural Somerset County, about 20 miles away. It was like an atomic bomb hit, said John Walsh, 72, who heard the crash and drove to the site while still in his bathrobe. When I got there, the plane was obliterated. You couldnt see the cockpit or the wings or nothing. Mark Stahl was listening to reports about the World Trade Center attacks on the radio when he heard Flight 93 crash. In nearby Shanksville, the crash sent people running to their doors, and the fire whistles began blowing. I didnt know what to think, Stahl said. It was shocking. United CEO James Goodwin said the airline was sending a team to Pennsylvania to assist the investigation and provide assistance to family members. United said it had identified all passengers and crew and was notifying families. No names were released immediately. Todays events are a tragedy and our prayers are with everyone at this time, Goodwin said.