The search for Flight 370, which vanished early March 8 while flying from Kuala Lumpur to Beijing with 239 people on board, has now been expanded deep into the northern and southern hemispheres. Australian vessels scoured the southern Indian Ocean and China offered 21 of its satellites to help Malaysia in the unprecedented hunt.
With no wreckage found in one of the most puzzling aviation mysteries of all time, relatives of those on the Boeing 777 have been left in an agonizing limbo.
Investigators say the plane was deliberately diverted during its overnight flight and flew off-course for hours. They haven't ruled out hijacking, sabotage, or pilot suicide, and they are checking the backgrounds of the 227 passengers and 12 crew members, as well as the ground crew, to see if links to terrorists, personal problems or psychological issues could be factors.
Malaysian Defense Minister Hishammuddin Hussein said finding the plane was still the main focus, and he did not rule out that it might be discovered intact.
"The fact that there was no distress signal, no ransom notes, no parties claiming responsibility, there is always hope," Hishammuddin said at a news conference.
Malaysian Airlines CEO Ahmad Jauhari Yahya said an initial investigation indicated that the last words heard from the plane by ground controllers - "All right, good night" - were spoken by the co-pilot, Fariq Abdul Hamid. Had it been a voice other than that of Fariq or the pilot, Zaharie Ahmad Shah, it would have clearest indication yet of something amiss in the cockpit before the flight went off-course.
Malaysian officials said earlier that those words came after one of the jetliner's data communications systems - the Aircraft Communications Addressing and Reporting System - had been switched off, suggesting the voice from the cockpit may have been trying to deceive ground controllers.
However, Amhad said that while the last data transmission from ACARS - which gives plane performance and maintenance information - came before that, it was still unclear at what point the system was switched off, making any implications of the timing murkier.
The new information opened the possibility that both ACARS and the plane's transponders, which make the plane visible to civilian air traffic controllers, were turned off at about the same time. It also suggests that the message delivered from the cockpit could have preceded any of the severed communications.
Authorities confiscated a flight simulator from the pilot's home Saturday and also visited the home of the co-pilot in what Malaysian police chief Khalid Abu Bakar initially said was the first police visits to those homes.
But the government, which has come under criticism abroad for missteps and foot-dragging in releasing information, issued a statement Monday contradicting that account, saying police first visited the pilots' homes as early as March 9, the day after the flight disappeared.
Although Malaysian authorities requested that all nations with citizens aboard the flight conduct background checks on them, it wasn't clear how thoroughly they were doing such checks at home. The father of a Malaysian aviation engineer aboard the plane said police had not approached anyone in the family about his 29-year-old son, Mohamad Khairul Amri Selamat, though he added that there was no reason to suspect him.
"It is impossible for him to be involved in something like this," said Selamat Omar, 60. "He is a good boy. ... We are keeping our hopes high. I am praying hard that the plane didn't crash and that he will be back soon."
French investigators arriving in Kuala Lumpur to lend expertise from the two-year search for an Air France jet that crashed in the Atlantic Ocean in 2009 said they were able to rely on distress signals. But that vital tool is missing in the Malaysia Airlines mystery because the flight's communications were deliberately silenced ahead of its disappearance, investigators say.
"It's very different from the Air France case. The Malaysian situation is much more difficult," said Jean Paul Troadec, a special adviser to France's aviation accident investigation bureau.
Malaysia's government sent diplomatic cables to all countries in the search area, seeking more planes and ships for the search, as well as to ask for any radar data that might help.
The search involves 26 countries and initially focused on seas on either side of Peninsular Malaysia, in the South China Sea and the Strait of Malacca.
Over the weekend, Prime Minister Najib Razak said investigators determined that a satellite picked up a faint signal from the aircraft about 7½ hours after takeoff. The signal indicated the plane would have been somewhere on a vast arc stretching from Kazakhstan in Central Asia to the southern reaches of the Indian Ocean.
Hishammuddin said Monday that searches in both the northern and southern stretches of the arc had begun, and that countries from Australia to China in the north and Kazakhstan in the west had joined the hunt.
Had the plane gone northwest to Central Asia, it would have crossed over countries with busy airspace, and some experts believe it more likely would have gone south, although Malaysian authorities are not ruling out the northern corridor and are eager for radar data that might confirm or rule out that route.
The northern corridor crosses through countries including China, India and Pakistan - all of which have said they have seen no sign of the plane. China, where two-thirds of the passengers were from, is providing several planes and 21 satellites for the search, Premier Li Keqiang said in a statement.
"Factors involved in the incident continue to multiply, the area of search-and-rescue continues to broaden, and the level of difficulty increases, but as long as there is one thread of hope, we will continue an all-out effort," Li said.
Indonesia focused on Indian Ocean waters west of Sumatra, air force spokesman Rear Marshall Hadi Tjahjanto said.
Australia agreed to Malaysia's request to take the lead in searching the southern Indian Ocean with four Orion maritime planes that also would be joined by New Zealand and U.S. aircraft, Australian Prime Minister Tony Abbott said.
"Australia will do its duty in this matter," Abbott told Parliament. "We will do our duty to the families of the 239 people on that aircraft who are still absolutely devastated by their absence, and who are still profoundly, profoundly saddened by this as yet unfathomed mystery."
The southern Indian Ocean is the world's third-deepest and one of the most remote stretches of water, with little radar coverage.
Associated Press writers Chris Brummitt, Jim Gomez and Eileen Ng in Kuala Lumpur, Kristen Gelineau in Sydney, Australia, Christopher Bodeen in Beijing and Niniek Karmini in Jakarta, Indonesia, contributed to this report.