Investigators are considering various possibilities, including mechanical or electrical failure, hijacking, sabotage, terrorism or issues related to the mental health of the pilots or someone else on board.
The search for the wreckage and the plane's flight data and cockpit voice recorders could take years because the ocean can extend to up to 23,000 feet deep in some parts. It took two years to find the black box from an Air France jet that went down in the Atlantic Ocean on a flight from Rio de Janeiro to Paris in 2009, and searchers knew within days where the crash site was.
There is a race against the clock to find Flight 370's black boxes, whose battery-powered "pinger" could stop sending signals within two weeks. The batteries are designed to last at least a month.
Retired Marine Col. Steven Ganyard, an ABC News aviation consultant, believes submarines should be involved with the search.
“They have computers on board, they have very sensitive listening capabilities, they’re designed to do these sorts of things,” Ganyard said.
“We’ve got to get more listening capability into the area because we’re racing against the clock, racing against that battery running out in those pingers.”
ABC News' Rebecca Lee and The Associated Press contributed to this report.