"Malaysian authorities have refocused their investigation on crew and passengers aboard," he said. "Evidence is consistent with someone acting deliberately from inside the plane."
"Despite media reports that the plane was hijacked, we are investigating all major possibilities on what caused MH370 to deviate," he added.
The Boeing 777-200 disappeared March 8 on a flight from Kuala Lumpur, the capital of Malaysia, to Beijing with 239 people on aboard.
Shortly after Najib's news conference, a source close to the investigation told CNN the home of pilot Zaharie Ahmad Shah, 53, had been searched by Malaysian police.
There was no search conducted as of early Saturday morning at the home of co-pilot Fariq Ab Hamid, 27.
Najib also announced a shift in the search area for the missing plane.
"The plane's last communication with the satellite was in one of two possible corridors: a northern corridor stretching approximately from the border of Kazakhstan and Turkmenistan to northern Thailand, or a southern corridor stretching approximately from Indonesia to the southern Indian Ocean," he said.
"Based on new satellite information, we can say with a high degree of certainty that the Aircraft Communications Addressing and Reporting System, or ACARS, was disabled just before the aircraft reached the East Coast of peninsular Malaysia," the prime minister added. "Shortly afterward, near the border between Malaysian and Vietnamese air traffic control, the aircraft's transponder was switched off. From this point onwards, the Royal Malaysian Air Force primary radar showed that an aircraft -- which was believed but not confirmed to be MH370 -- did turn back."
[New York Times]
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