Officials revealed that the final words from the cockpit — "All right, good night" — were spoken to air traffic controllers after the plane's data communication system had been partially disabled. The pilots did not mention any trouble on board, suggesting they may have been misleading ground control.
Asked what the significance of this was, air force Maj. Gen. Affendi Buang told reporters: "This will tell you something ... because this is something not normal that the pilot would do."
Affendi said he did not know whether it was the pilot or co-pilot who spoke to air traffic controllers. That uncertainty also opened the possibility that someone else spoke those words, though he did not mention this scenario.
The Boeing 777 went missing less than an hour into a March 8 flight from Kuala Lumpur to Beijing as it entered Vietnamese airspace. On Saturday, Malaysia's government confirmed that the plane was deliberately diverted and may have flown as far north as Central Asia, or south into the vast reaches of the Indian Ocean.
Given the expanse of land and water that might need to be searched, the wreckage of the plane might take months — or longer — to find, or might never be located. Establishing what happened with any degree of certainty will likely need key information, including cockpit voice recordings, from the plane's flight data recorders.
The search area now includes 11 countries the plane might have flown over, Malaysia's acting transport minister, Hishammuddin Hussein, said Sunday, adding that the number of countries involved in the operation had increased from 14 to 25.
"The search was already a highly complex, multinational effort. It has now become even more difficult," he said at a news conference.
The search effort initially focused on the relatively shallow waters of the South China Sea and the Strait of Malacca, where the plane was first thought to be. Hishammuddin said he had asked governments to hand over sensitive radar and satellite data to try and help get a better idea of the plane's final movements.
"It is our hope with the new information, parties that can come forward and narrow the search to an area that is more feasible," he said.
Malaysia is leading the search for the plane and the investigation into its disappearance.
In the United States, Dan Pfeiffer, senior adviser to President Barack Obama, told NBC's "Meet the Press" that the FBI was supporting the criminal probe.
Rep. Peter King, who is chairman of the House Homeland Security subcommittee on counterterrorism and intelligence, said on ABC's "This Week" that so far "there's nothing out there indicating it's terrorists."
Investigators are trying to answer these questions: If the two pilots were involved in the disappearance, were they working together or alone, or with one or more of the passengers or crew? Did they fly the plane under duress or of their own volition? Did one or more of the passengers manage to break into the cockpit, or use the threat of violence to gain entry and then pilot the plane? And what possible motive could there be for flying off with the plane?
Malaysia's police chief, Khalid Abu Bakar, said he requested countries with citizens on board the plane to investigate their background, no doubt looking for any ties to terrorist groups, aviation skills or evidence of prior contact with the pilots. He said that the intelligence agencies of some countries had already done this and found nothing suspicious, but that he was waiting for others to respond.
The government said police searched the homes of both of the pilots on Saturday, the first time they have done so since the plane went missing. Asked why it took them so long, Khalid said authorities "didn't see the necessity in the early stages."
Khalid said police confiscated the elaborate flight simulator that one of the pilots, Zaharie Ahmad Shah, had built in his home and reassembled it in their offices to study it for clues.
Zaharie, 53, who has three grown children and one grandchild, had previously posted photos online of the simulator, which was made with three large computer monitors and other accessories. Earlier this week, the head of Malaysia Airlines said this was not in itself cause for any suspicion.
Malaysian police are also investigating engineers and ground staff who may have had contact with the plane before it took off, Khalid said.
With the plane's transponder and its data transmission system disabled, experts from around the world are trying to use data from a satellite system not designed to track a plane's location to see where the aircraft might be.
The last satellite "ping," recorded at 8:11 a.m. — 7 hours and 31 minutes after the plane took off from Kuala Lumpur — placed it somewhere in a huge arc as far north as Kazakhstan in Central Asia or far into the southern Indian Ocean, posing awesome challenges for efforts to recover the aircraft and flight data recorders vital to solving one of modern aviation's biggest mysteries.
While most aviation experts believe the plane has crashed, there is a very small possibility it may have landed somewhere and be relatively intact. Affendi, the air force general, and Hishammuddin, the acting transport minister, said it was possible for the plane to "ping" when it was on the ground if its electrical systems were undamaged.
Australia said it was sending one of its two AP-3C Orion aircraft involved in the search to remote islands in the Indian Ocean at Malaysia's request. The plane will search the north and west of the Cocos Islands, a remote Australian territory with an airstrip about 1,200 kilometers (745 miles) southwest of Indonesia, military chief Gen. David Hurley said.
Given that the northern route the plane may have taken would take it over countries with busy airspace, most experts say the person in control of the aircraft would more likely have chosen the southern route. The southern Indian Ocean is the world's third-deepest and one of the most remote stretches of water in the world, with little radar coverage.
Robinhood Simanjutak, whose sister Surtidahlia, 50, was aboard the flight, said police in Indonesia had contacted him for help in the investigation. "Every night, I pray that I will be able to meet Surti again. I have high hopes," he said. "We cannot make any conclusion until we know what had really happened. It is beyond our control. We have to accept it."
Malaysian officials and aviation experts 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, putting one or both of the pilots high on the list of possible suspects.
Zaharie, the pilot, was a supporter of a Malaysian opposition political party that is locked in a bitter dispute with the government, according to postings on his Facebook page and a friend, Peter Chong, who is a party member.
Chong said that he last saw Zaharie a week before the pilot left on the flight for Beijing, and that they had agreed to meet on his return to organize a shopping trip for poor children.
"If I am on a flight, I would choose Captain Zaharie," he said. "He is dedicated to his job, he is a professional and he loves flying."