Investigators Saturday were hoping cockpit voice recorders recovered from two of the four hijacked flights would provide clues about the final moments of the doomed aircraft.
The voice recorder from United Flight 93, the hijacked plane that crashed in western Pennsylvania, was found buried 25 feet in the crash crater, according to FBI officials. The flight voice recorder was the second of "black boxes" to be found from United Flight 93. The data recorder was found Thursday.
Although the voice recorder appeared to be "in fairly good shape" according to FBI spokesman Bill Crowley, the National Transportation Safety Board was unable to get information from either of the black boxes because the computer chips inside the boxes were severely damaged.
The boxes will now be sent to their Seattle manufacturer, Allied Signal. In the past, the company has been successful in retrieving information from flight recorders despite similar damage.
One of the things investigators hope to learn is what target, if any, United Flight 93's hijackers hoped to hit. The flight, which left Newark, N.J. with 45 people on board, bound for San Francisco, turned back east over Cleveland and flew erratically before going down near rural Shanksville, Penn.
Some have speculated the hijackers planned to target Camp David.
Passengers on the plane made cell phone calls to relatives suggested they planned to fight back against their hijackers and prevent the Boeing 757 from being crashed into buildings in order to destroy the structures, as were the planes that destroyed the World Trade Center.
"The passengers on that plane decided to fight back their hijackers. They undoubtedly saved hundreds, if not thousands, of lives in the process. They sacrificed themselves for others — the ultimate sacrifice," Pennsylvania Gov. Tom Ridge said at a vigil for the victims on Friday.
Ridge called the crater created by the plane's impact "a monument to heroism."
About 3,000 attended the vigil, including 80 friends and relatives of the victims.
Meanwhile, investigators tell Fox News they are getting "good, solid readings" from American Airlines Flight 77's data recorder. That plane crashed into the Pentagon, killing a total of 188 people in Washington — a combination of military and civilian employees on the ground and the passengers in the plane.
According to data on the recorder, the plane was going 345 miles per hour when it crashed at about 9:30 Tuesday morning. Investigators also say the recorder has speed an altitude information for the plane's entire flight.
The plane's voice recorder was also recovered, but National Transportation Safety Board officials say it was too damaged in the fire to obtain any information from it.
Fox News' Rita Cosby and the Associated Press contributed to this report.