Until yesterday, the investigation was in the hands of the National Transportation and Safety Board (NTSB), which is empowered to determine the cause of air crashes within US jurisdiction.
The transfer of authority was the first concrete indication that sabotage of some sort, whether by a member of the crew or a passenger, was suspected. The plane plunged into the ocean off Massachusetts in the early hours of 31 October, with the loss of all 217 people on board.
The passengers' last minutes appeared to be a nightmare roller-coaster ride at almost supersonic speed, in which they experienced first weightlessness and then exceptionally heavy pressure in a cabin that was decompressing.
To that yesterday was added the speculation of a desperate cockpit struggle for control of the plane as it plummeted. Attention was focusing on what appeared to be a short Islamic prayer uttered by one of the pilots. The words, according to investigators, are quite distinct and were confidently interpreted and identified from the Arabic.
What was not clear, however, was whether they were uttered immediately before the autopilot was disconnected, or shortly afterwards. The timing is critical, as it would indicate whether the words were spoken to accompany an intentional act, or whether they were an expression of despair in an emergency that already appeared fatal.
In order to establish the timing, investigators were synchronising the evidence from the two black boxes, the flight data recorder which was recovered from the ocean bed last week, and the cockpit voice recorder, which was recovered on Sunday. According to earlier reports, the flight data recorder contains detailed information about the mechanical aspects of the flight until its final, precipitate plunge.
Investigators have been more reticent about the quality and duration of the information on the cockpit voice recorder, other than signalling late on Monday that it could contain evidence of a crime.
The inquiry also has two types of radar evidence: from civilian air-traffic control and from naval radar, which are said to back up the evidence on the flight-data recorder.
Air-traffic controllers received no communication from the plane to suggest that it was in any difficulty. The last voice contact was a standard confirmation half an hour into the flight that the plane had reached a cruising altitude of 33,000 feet above the Atlantic en route to Cairo.
What happened between then and the catastrophic plunge into the Atlantic just minutes later is what the investigators were still trying to find out yesterday.
With evidence from the flight data recorder apparently ruling out mechanical failure as the cause, the inquiry focused on human error or deliberate sabotage.
Shortly after the NTSB broached the possibility that the investigation could be passed to the FBI, a source close to the investigation told reporters that the voice tape suggested the pilot had left the cabin shortly before the autopilot was disconnected. There was the sound, he said, like the cockpit door opening and closing. The pilot appeared to return when the first, rapid, descent was already in progress. Thereafter, he said: "There is some evidence they are working at cross-purposes."
That evidence came apparently from the flight data recorder which showed the two elevator mechanisms, which adjusted height and direction, being manipulated in opposite directions before the plane's two engines were switched off. Both were actions that the Boeing aircraft company said could be accomplished only manually, and so deliberately. It was the manual switching off of the engines that apparently caused the plane to rise 7,000 feet before its final, almost vertical, dive 24,000 feet into the ocean. Officials have at no stage suggested that the engines might have stalled.
Until late on Monday, the voice tape evidence was said to show the pilot and co-pilot chatting "as pals", with no sign of either discord or panic. While that may have been true of the early minutes of the flight, this version was superseded by reports that the tape contained sounds of the cockpit door being opened and closed several times. However, some of the sounds on the tape were said to be hard to identify with certainty because of background noise.
It was Arabic interpreters, sources said, who immediately identified the prayer spoken by one of the pilots. The NTSB had earlier made known its intention of also asking former colleagues of the two pilots to listen to the tape as they might put a different gloss on exchanges from their knowledge of how the men worked.
While the opening and closing of doors and the actions made manually would not exclude the possibility of a hijacking, terrorism has hardly been mention as a plausible theory. The CIA, involved in the investigation from the start, apparently offered no evidence of any plausible threat and there were no claims of responsibility.
That there might have been a bomb on board, or a missile strike, also seemed unlikely after investigators found no evidence of fire on debris that was recovered.
The head of the NTSB, Jim Hall, said that from what he had seen of the wreckage, it resembled that of the ValuJet DC-9 plane that crashed into the Florida Everglades in 1996 more than that of TWA800, the jumbo jet that crashed off Long Island in the same year, for which terrorism was an early and persistent theory.
While the announcement that control of the investigation was to be handed to the FBI made clear that sabotage was a possible, if not yet the favoured, explanation, other causes could not be ruled out.
Competition between the NTSB and the FBI for control of air crash investigations - and the money that goes with them - is a fact of life in the United States, which can hamper the efforts of both.
But the open antagonism between the two agencies which so hobbled the TWA800 investigation and, to a lesser extent, the investigation into last year's crash of Swissair 111 off Nova Scotia, had not been much in evidence this time.
The rescue, recovery and publicity operation that followed the EgyptAir disaster appeared streamlined and under the single command of the NTSB.
The privacy of relatives of the crash victims was successfully protected, and most of the initial information about the crash came through official channels, eliciting praise that the lessons of TWA 800 had been learned.
This makes the formal transfer of responsibility from one agency to the other a more significant pointer towards sabotage than it would otherwise have been.Reuse content