Experts say 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 divert the plane for some reason — possibly even to commit suicide. Piracy and hijacking also have been cited as possible explanations.
Najib stressed that investigators were looking into all possibilities.
"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. Malaysian police have said they are looking at the psychological state, family life and connections of pilot Zaharie Ahmad Shah, 53, and co-pilot Fariq Abdul Hamid, 27. They released no details on their investigation so far.
Zaharie, who joined Malaysia Airlines in 1981 and had more than 18,000 hours of flying experience, was known as an avid aviation enthusiast who had set up an elaborate flight simulator at home.
Fariq was contemplating marriage after having just graduated to the cockpit of a Boeing 777. He has drawn scrutiny after the revelation that he and another pilot invited two female passengers to sit in the cockpit during a flight in 2011.
Two-thirds of the plane's passengers were Chinese, and China's government has been under pressure to give anxious relatives firm news of the aircraft's fate. Beijing's state media expressed irritation Saturday at what it described as Malaysia's foot-dragging in releasing information about the investigation and the search.
At a hotel near Beijing's airport, some relatives said they felt deceived at not being told earlier about the plane emitting signals for 7 ½ hours. "We are going through a roller coaster, and we feel helpless and powerless," said a woman, who declined to give her name.
Najib, at his news conference, said he understood the need for families to receive information, but that his government wanted to release only fully corroborated details.
The missing Malaysia Airlines flight departed Kuala Lumpur at 12:40 a.m. heading toward Beijing. 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, saying that a team of Malaysian, U.S. and British aviation investigators concurred in the findings so far.
Although the aircraft was flying virtually blind to air traffic controllers at this point, onboard equipment continued to send "pings" to satellites.
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 ACARS 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.