The FBI director, Mr Robert Mueller, confirmed yesterday that one of the black boxes from the passenger plane that smashed into the Pentagon last week is unreadable.
"There were two recorders found at the Pentagon site, the flight data recorder and the voice recorder, and unfortunately we're unable to obtain anything off the voice recorder because it was so badly burned in that crash," he said.
The FBI chief said the two black boxes retrieved from the Pennsylvania plane, United Airlines flight 93, had been recovered and were being examined by the FBI and the National Transportation Safety Board.
No flight recorders have been recovered from the jets that flew into the twin towers, he said.