The first turn to the west that diverted the missing Malaysia Airlines plane from its planned flight path from Kuala Lumpur to Beijing was carried out through a computer system that was most likely programmed by someone in the plane's cockpit who was knowledgeable about airplane systems, according to senior American officials.
Instead of manually operating the plane's controls, whoever altered Flight 370's path typed seven or eight keystrokes into a computer on a knee-high pedestal between the captain and the first officer, according to officials. The Flight Management System, as the computer is known, directs the plane from point to point specified in the flight plan submitted before each flight. It is not clear whether the plane's path was reprogrammed before or after it took off.
The fact that the turn away from Beijing was programmed into the computer has reinforced the belief of investigators — first voiced by Malaysian officials — that the plane was deliberately diverted and that foul play was involved. It has also increased their focus on the plane's captain and first officer.
Prime Minister Najib Razak of Malaysia told reporters on Saturday that his government believed that the plane had been diverted, because its transponder and other communications devices had been manually turned off several minutes apart. American officials were told of the new information over the weekend.
But Malaysian authorities on Monday reversed themselves on the sequence of events they believe took place on the plane in the crucial minutes before ground controllers lost contact with it early on March 8. They said it was the plane's first officer — the co-pilot — who was the last person in the cockpit to speak to ground control. And they withdrew their assertion that another automated system on the plane called Aircraft Communications Addressing and Reporting System, or Acars, had already been disabled when the co-pilot spoke.
The Flight Management System reports its status to Acars, which in turn transmits information back to a maintenance base, according to an American official. Acars ceased to function about the same time that oral radio contact was lost and the airplane's transponder also stopped, fueling suspicions that foul play was involved in the plane's disappearance.
Investigators are scrutinizing radar tapes from when the plane first departed Kuala Lumpur because they believe the tapes would show that after the plane first changed its course, it passed through several pre-established "waypoints," which are like virtual mile markers in the sky. That would suggest that the plane was under control of a knowledgeable pilot, because passing through those points without using the computer would have been unlikely.
(Read more: MH370: Timeline of latest developments)
According to investigators, it appears that a waypoint was added to the planned route. Pilots do that in the ordinary course of flying if air traffic controllers tell them to take a different route, to avoid weather or traffic. But in this case, the waypoint was far off the path to Beijing.
Whoever changed the plane's course would have had to be familiar with Boeing aircraft, though not necessarily the 777 — the type of plane that disappeared. American officials and aviation experts said it was far-fetched to believe that a passenger could have reprogrammed the Flight Management System.
Normal procedure is to key in a five-letter code — gibberish to non-aviators — that is the name of a waypoint. A normal flight plan consists of a series of such waypoints, ending in the destination airport. For an ordinary flight, waypoints can be entered manually or uploaded into the F.M.S. by the airline.
One of the pilots keys in a waypoint on a separate screen known as a scratchpad, and after confirming that it has no typographical errors, pushes another button to move it into the sequence already in the flight plan. Normal practice is to orally confirm the waypoint with the other pilot, then push another button to instruct the airplane to go there. With the change in course, the plane would bank at a comfortable angle, around 20 degrees, and make the turn. Passengers would not feel anything unusual.
ABC News reported on Sunday that the programmed turn had led investigators to believe that it was being controlled by the pilot or hijackers.
One American safety expert, John Cox, a former airline union safety official, said that someone taking such pains to divert the plane does not fit the pattern of past cases when pilots intentionally crashed and killed everyone on board.
"There's an inconsistency in what we've seen historically," he said, comparing the disappearance of Flight 370 with two murder-suicides, of an Egyptair flight off Nantucket Island in 1999 and a SilkAir jet in Indonesia in 1997. In those crashes, he said, the pilot involved simply pushed the nose of the plane down and flew into the water. The authorities searched the homes of the pilots in Kuala Lumpur on Saturday, seizing a flight simulator that one of them had in his home.
In an effort to determine whether the pilot had practiced taking down the plane, the authorities have reassembled the simulator for experts to examine. American investigators would like access to the flight simulator and any other electronic information seized from the pilots, but as of Monday night they had not been given access to those materials.
Meanwhile, as the search for the missing Boeing 777 jet stretched into a 10th day, two of the nations helping in the hunt, Australia and Indonesia, agreed to divide between them a vast area of the southeastern Indian Ocean, with Indonesia focusing on equatorial waters and Australia beginning to search farther south for traces of the aircraft. To the north, China and Kazakhstan checked their radar records and tried to figure out whether the jet could have landed somewhere on their soil.