The missing Malaysia Airlines jetliner sent out its last signal about 7 1/2 hours after take off, which means the plane could have have ended up as far as Kazakhstan or deep in the southern Indian Ocean, the Malaysian prime minister said.
"In view of this latest development, the Malaysian authorities have refocused their investigation into the crew and passengers on board," Malaysian Prime Minister Najib Razak said at a press conference, stressing they were still investigating all possibilities as to why the plane deviated so drastically from its original flight path.
"Clearly the search for (Flight) MH370 has entered a new phase," Najib told a televised news conference.
Najib also said that authorities are now trying to trace the airplane across two possible "corridors" -- a northern corridor from the border of Kazakstan and Turkmenistan through to northern Thailand, and a southern corridor from Indonesia to the southern Indian Ocean. 
The prime minister said that searching in the South China Sea, where the plane first lost contact with air traffic controllers, would be ended. He said the new search corridors were based on the latest available satellite data.
Najib also confirmed that Malaysian air force defense radar picked up traces of the plane turning back westward, crossing over Peninsular Malaysia into the northern stretches of the Strait of Malacca. Authorities previously had said this radar data could not be verified.
He said the jetliner's "movements are consistent with the deliberate action of someone on the plane," BBC News reported.
The Malaysian Airlines Flight 370 was carrying 239 people when it departed for an overnight flight from Kuala Lumpur to Beijing at 12:40 a.m. March 8. The plane's communications with civilian air controllers were severed about 1:20 and the plane went missing in one of the most puzzling mysteries in modern aviation history.
The prime minister said investigators have not ruled out any possible cause for the plane's disappearance, despite an earlier report by The Associated Press that investigators had concluded that one of the pilots or someone else with flying experience hijacked the jetliner.
A Malaysian government official involved in the investigation told the AP on Saturday that no motive has been established, and it is not yet clear where the plane was taken. The official spoke on condition of anonymity because he was not authorized to brief the media. 
The official said that hijacking was no longer a theory. "It is conclusive," he said.
He said evidence that led to the conclusion were signs that the plane's communications were switched off deliberately, data about the flight path and indications the plane was steered in a way to avoid detection by radar. 
Malaysian officials have said radar data suggest it may have turned back and crossed back over the Malaysian peninsula westward, after setting out toward the Chinese capital. 
Malaysian police have previously said they were checking whether any passengers or crew had personal or psychological problems that might offer clues to why the plane vanished, along with the possibility of a hijacking, sabotage or mechanical failure.
Earlier Friday, a senior U.S. official told Fox News that the search effort would be broadened deep into the Indian Ocean, based on new intelligence assessments that there is a "higher probability" the aircraft went down in that region. 
As a consequence of shared U.S.-Malaysian intelligence assessments, the U.S. Navy destroyer USS Kidd is expected to expand its search into a southern quadrant of the ocean, while Indian authorities will cover a northern quadrant. 
The development comes as authorities speculate that the disappearance may have been an "act of piracy,” and more evidence suggests the plane was diverted by a skilled pilot before it vanished, U.S. and Malaysian officials familiar with the investigation said Friday.
Key evidence for "human intervention" in the plane's disappearance is that contact with its transponder stopped about 12 minutes before a messaging system quit, an unidentified American official told the Associated Press. The official -- also not authorized to speak publicly -- said it's also possible the plane may have landed somewhere. 
A source familiar with the investigation but not authorized to speak on the record told Fox News that flight 370 continued to send "periodic pushes" of data after the transponder went dark for about four hours after contact was lost with the aircraft, suggesting the jet continued to fly. This was described to Fox News as signals data that, in isolation, would not provide location data.  
While the systems were no longer transmitting maintenance data, the satellite communication link was still active. Once an hour, the system sent out a “handshake” -- a form of reset, like a cell phone searching for an antenna tower. 
The “handshake” allows the satellite to work out how much tilt or arc was needed to be in range of the plane's signal. It therefore provides a scope or range for the aircraft, but it does not provide altitude, speed or location.
If the plane had disintegrated during flight or had suffered some other catastrophic failure, all signals — the pings to the satellite, the data messages and the transponder — would be expected to stop at the same time.
Analysis of the Malaysia flight data suggests the plane diverted from its intended northeast route from Kuala Lumpur to Beijing and instead flew west, using airline flight paths normally taken to the Middle East and Europe, Reuters reported Friday. 
This points to the theory that the plane was being flown by the pilots or possibly someone familiar with those routes, according to sources in the Reuters report.
Malaysian police searched the home of the pilot of the missing jetliner Saturday after it was revealed at a press conference that the flight tracker was turned off from inside the plane’s cockpit on the day it disappeared. 
Details such as these are leading investigators to sharpen their focus the possibility of sabotage, The Wall Street Journal reported late Friday.
Mike Glynn, a committee member of the Australian and International Pilots Association, said he considers pilot suicide to be the most likely explanation for the disappearance, as was suspected in a SilkAir crash during a flight from Singapore to Jakarta in 1997 and an EgyptAir flight from Los Angeles to Cairo in 1999.  
"A pilot rather than a hijacker is more likely to be able to switch off the communications equipment," Glynn said. "The last thing that I, as a pilot, want is suspicion to fall on the crew, but it's happened twice before."
"What we can say is we are looking at sabotage, with hijack still on the cards," said that source, a senior Malaysian police official.  
The current search involves 14 countries, 43 ships and 58 aircraft.
Fox News' Catherine Herridge, Justin Fishel and The Associated Press contributed to this report.