Malaysia's Prime Minister Najib Razak says the communications systems of missing Malaysia Airlines flight MH370 were deliberately disabled, and that it turned back across northern Malaysia and flew into the Indian Ocean.
According to satellite and radar evidence, he said, the plane then changed course and could have continued flying for a further seven hours.
He said the "movements are consistent with the deliberate action of someone on the plane".
The plane disappeared a week ago with 239 people on board.
The Kuala Lumpur-Beijing flight last made contact with air traffic control over the South China Sea to the east of Malaysia, about one hour after take-off.
Mr Najib told a news conference that new satellite evidence shows "with a high degree of certainty" that the aircraft's communications systems were disabled and then it changed course, flying back over Malaysia towards India.
Satellite signals continued to be picked up from the plane some seven hours after it lost radar contact.
Mr Najib said the authorities were now trying to trace the plane across two possible corridors: from the border of Kazakhstan and Turkmenistan through to northern Thailand; or south from Indonesia to the southern Indian Ocean.
The investigation, he said, had "entered a new phase", and the search of the South China Sea had been discontinued.
Addressing reports that the plane had been hijacked, he said only "we are still investigating all possibilities as to what caused MH370 to deviate".
Malaysian police began searching the home of the senior pilot of the missing Malaysian airliner, with officers arriving at the home of the captain, 53-year-old Zaharie Ahmad Shah, shortly after the Prime Minister's news conference.
The families on board the flight have endured an agonising wait for news since the plane disappeared on 8 March.
An extensive search of the seas around Malaysia - involving 14 countries, 43 ships and 58 aircraft - have proved fruitless.
The BBC reports that Mr Najib's news conference went some way to addressing the speculation that had begun circulating in local media that the plane had been hijacked and had somehow landed intact.
The Boeing 777 has one of the best safety records of any jet, and authorities have struggled to find any indication of what went wrong, keeping alive a host of theories that include a mid-air explosion, terrorist act, catastrophic technical failure or rogue missile strike.
- 153 Chinese including a delegation of artists
- 38 Malaysians including an official who was due to start a job at a branch office in Beijing
- 2 Iranians using false passports in a bid to seek asylum in Europe
- 3 Americans including an IBM executive who had recently relocated to Kuala Lumpur
- 2 Canadians returning to Beijing after a business trip
- 7 Indonesians, 6 Australians, 5 Indians and 4 French
- 2 each from New Zealand and Ukraine;
- one each from Russia, Taiwan and Netherlands
Waiting the hardest part, says NZ wife
The wife of one of the two New Zealand passengers on the missing plane says she's still hoping for the best, but admits the situation doesn't look good.
Paul Weeks, 38, was flying from Perth to a mining job in Mongolia. His wife, Danica, said the lack of information is hard on her and the couple's two sons.
"That's the toughest part every day waking up and just looking on the news and seeing that there's nothing and there's no calls from Malaysia to say we've found something. Every day it just seems like it's an eternity, it's an absolute eternity."
Ms Weeks said her husband is her best friend and "the most amazing father".
The other New Zealander is Ximin Wang, 50, who is described on the electoral roll as a student.
New Zealand Prime Minister John Key said on Friday that an Air Force Orion would continue to assist efforts to find the plane for as long as it is needed.