The investigation into the disappearance of the Malaysia Airlines jet with 239 on board is now focused on the crew and passengers.
Malaysian Prime Minister Najib Razak said Saturday there is a "high degree of certainty" that someone who knew what he was doing deliberately turned off the communications system.
He said it was switched off just before the jet turned westward, away from its flight path en route to Beijing.
But Mr. Razak stopped short of saying the plane was hijacked and said investigators are looking at all scenarios.
Witnesses in Kuala Lumpur said police went to the homes of the pilot and co-pilot Saturday. Officials had no comment.
The frustrating search for Flight 370 now ranges from Kazakhstan to the southern Indian Ocean. Investigators say the plane had enough fuel to fly for several hours after disappearing from radar Saturday, March 8.
Ships and planes from 15 countries are looking for the missing Boeing 777. It was flying from Kuala Lumpur to Beijing when it disappeared. About two-thirds of the people on board were Chinese. Other passengers included Europeans and Americans.