"Deliberate action" diverted the missing Malaysia Airlines plane after its communications were cut, according to Malaysia's Prime Minister.
Najib Razak was speaking shortly before it was reported the pilot's house was being searched by police.
At a packed news conference, Mr Razak stopped short of saying flight MH370 had been hijacked, despite a government official earlier saying this was the case.
But it emerged that satellite data suggested the jet had continued flying for hours after its last detection by civilian radar.
Mr Razak stressed "all possibilities" were being looked into to try to resolve the mystery surrounding the flight's fate. It left Kuala Lumpur last Saturday bound for Beijing with 239 people on board.
Shortly after he finished speaking, officers began searching the house of 53-year-old pilot Zaharie Ahmad Shah, Reuters reported. The official said they had gone to collect evidence that could help with the investigation.
The final satellite communication with the Boeing 777 came more than six and a half hours after it disappeared from civilian radar at 1.30am local time on March 8 (5.30pm UK time the previous day).
In this period the aircraft changed direction and passed back over the Malaysian peninsula towards the Indian Ocean.
Data confirmed an unidentified aircraft that later appeared on military radar off Malaysia's west coast before going out of range at 2.15am was flight MH370.
"Up until the point at which it left military primary radar coverage, these movements are consistent with deliberate action by someone on the plane," Mr Razak said.
The search, which involves 14 countries, 43 ships and 58 aircraft, now encompasses two "corridors".
The first is a northern corridor from the border of Kazakhstan and Turkmenistan through to northern Thailand, and the second is a southern corridor from Indonesia to the southern Indian Ocean.
The plane first lost contact with air traffic controllers in the South China Sea, and Mr Razak said the search there would be called off.
He said: "Clearly the search for MH370 has entered a new phase. We hope this new information brings us one step closer to finding the plane."
Most of the passengers on board were Chinese, and relatives gathered in Beijing to listen to Mr Razak's news conference. The address left many with unanswered questions , and some angrily suggested foul play.
Experts have told Sky News that a hijacking scenario is looking "increasingly likely" .
Earlier, a source close to the investigation said satellite pulses picked up from the flight show it may have been flying off-course for several hours before running out of fuel over the Indian Ocean.
Analysis of military radar tracking and pulses has provided two different theories as to what may have happened to the plane, the unnamed source said.
The electronic signals are believed to have been transmitted for up to five hours after ground control lost contact with the aircraft, according to Sky sources.
The signals are 'pings' sent by the plane to confirm it is still there and to allow the network to determine its position.
The source close to the investigation said the most likely possibility is that after travelling northwest, the Boeing 777 made a sharp turn to the south, over the Indian Ocean where officials think, based on the data, it flew until it ran out of fuel and crashed into the sea.
The other interpretation is the plane continued to fly to the northwest and headed over Indian territory.
But the source said it was believed unlikely the plane flew for any length of time over India because that country has strong air defence and radar coverage which should have allowed authorities to see the plane.