Final words from missing Malaysian plane came after systems shutdown

Given the expanse of land and water that might need to be searched, finding the wreckage could take months or longer. Or it might never be located. Establishing what happened with any degree of certainty will probably require evidence from cockpit voice recordings and the plane's flight-data recorders.

The search area now includes 11 countries the plane might have flown over, Hishammuddin said, 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," he said. "It has now become even more difficult."

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 to get a better idea of the plane's final movements.

With more information, he said, the search zone could be narrowed "to an area that is more feasible."

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, 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 will? Did one or more of the passengers manage to break into the cockpit or use the threat of violence to gain entry and then seize the plane? And what possible motive could there be for diverting the jet?