KUALA LUMPUR, Malaysia - Someone deliberately diverted Malaysia Airlines Flight 370 and shut down communications with the ground, and the jetliner continued flying for six hours, Malaysia's prime minister said Saturday. The announcement shifted the focus of the investigation to the crew and passengers on the plane, which has now been missing for more than a week.

Prime Minister Najib Razak's statement also meant the flight path of Malaysia Airlines Flight 370 to Beijing could have strayed as far as the southern Indian Ocean or northwest to Kazakhstan, complicating the work of search crews who already have been scouring vast stretches of ocean seeking the plane's 12-person crew and 227 passengers.

"Clearly the search for MH370 has entered a new phase," Najib said at a televised news conference. "It is widely understood that this has been a situation without precedent."

Experts have previously said that whoever disabled the plane's communication systems and then flew the jet must have had a high degree of technical knowledge and flying experience. One possibility they have raised was that one of the pilots wanted to commit suicide.

Najib stressed that investigators were looking into all possibilities as to why the Boeing 777 deviated so drastically from its original flight path, saying authorities could not confirm whether it was a hijacking. Earlier Saturday, a Malaysian official said the plane had been hijacked, though he added that no motive had been established and no demands had been made known.

"In view of this latest development, the Malaysian authorities have refocused their investigation into the crew and passengers on board," Najib told reporters, reading from a written statement but not taking any questions.

Police on Saturday went to the Kuala Lumpur homes of both the pilot and co-pilot of the missing plane, according to a guard and several local reporters. Authorities have said they will investigate the pilots as part of their probe, but have released no information about how they are progressing.

The plane departed for an overnight flight from Kuala Lumpur to Beijing at 12:40 a.m. on March 8. Its communications with civilian air controllers were severed at about 1:20 a.m., and the jet went missing - heralding one of the most puzzling mysteries in modern aviation history.

China, where the bulk of the passengers were from, expressed irritation over what it described as Malaysia's foot-dragging in releasing information about the search.

Investigators now have a high degree of certainty that one of the plane's communications systems - the Aircraft and Communications Addressing and Reporting System (ACARS) - was partially disabled before the aircraft reached the east coast of Malaysia, Najib said. Shortly afterward, someone on board switched off the aircraft's transponder, which communicates with civilian air traffic controllers.

Najib 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.

"These movements are consistent with deliberate action by someone on the plane," Najib said.

Although the aircraft was flying virtually blind to air traffic controllers at this point, onboard equipment continued to send "pings" to satellites.

U.S. aviation safety experts say the shutdown of communications systems makes it clear the missing Malaysia Airlines jet was taken over by someone who knew how the plane worked.

To turn off the transponder, someone in the cockpit would have to turn a knob with multiple selections to the "off" position while pressing down at the same time, said John Goglia, a former member of the U.S. National Transportation Safety Board. That's something a pilot would know, but it could also be learned by someone who researched the plane on the Internet, he said.

The Aircraft Communications Addressing and Reporting System has two aspects, Goglia said. The information part of the system was shut down, but not the transmission part. In most planes, the information section can be shut down by hitting cockpit switches in sequence in order to get to a computer screen where an option must be selected using a keypad, said Goglia, an expert on aircraft maintenance.

That's also something a pilot would know how to do, but that could also be discovered through research, he said.

But to turn off the other transmission portion of the ACARS, it would be necessary to go to an electronics bay beneath the cockpit. That's something a pilot wouldn't normally know how to do, Goglia said. The Malaysia plane's ACARS transmitter continued to send out blips that were recorded by satellite once an hour for four to five hours after the transponder was turned off. The blips don't contain any messages or data, but the satellite can tell in a very broad way what region the blips are coming from.

Malaysia's prime minister said the last confirmed signal between the plane and a satellite came at 8:11 a.m. - 7 hours and 31 minutes after takeoff. This was more than five hours later than the previous time given by Malaysian authorities as the possible last contact.

Airline officials have said the plane had enough fuel to fly for up to about eight hours.

"The investigations team is making further calculations which will indicate how far the aircraft may have flown after this last point of contact," Najib said.

He said authorities had determined that the plane's last communication with a satellite was in one of two possible arcs, or "corridors" - a northern one from northern Thailand through to the border of the Central Asian countries Kazakhstan and Turkmenistan, and a southern one from Indonesia to the southern Indian Ocean.

The northern route might theoretically have taken the plane through China, India, Pakistan, Afghanistan - which hosts U.S. military bases - and Central Asia, and it is unclear how it might have gone undetected. The region is also home to extremist Islamist groups, unstable governments and remote, sparsely populated areas.

Flying south would have put the plane over the Indian Ocean, with an average depth of 3,890 meters (12,762 feet) and thousands of kilometers (miles) from the nearest land mass.

Goglia said that if Malaysian military radar tracked the plane turning west, it then followed a standard route across the peninsula until it was several hundred miles (kilometers) offshore and beyond military radar range. Airliners generally keep to such highways in the sky to avoid colliding with other planes, but the routes are not straight lines, he said. That means it's likely someone was still guiding the plane, Goglia said.