Malaysia Airlines Flight 370

A wide search. And a ticking clock.

The box containing the flight-data and cockpit-voice recorders of missing Malaysia Airlines Flight 370 has batteries designed to keep it sending out pings for 30 days.

The search is now in its 12th day, covering a total area roughly the size of the continental United States. That leaves 18 days until the batteries are expected to run out.

Investigators hope the recorders may reveal vital information about why the passenger jet carrying 239 people veered dramatically off course and disappeared from radar screens. But they have to find them first.

Searchers from at least 26 countries have a formidable task in pinpointing the plane's location somewhere along two vast arcs, one stretching deep into the Asian landmass, the other far out into the Indian Ocean.

"The odds of finding the pinger are very slim," said Rob McCallum, an ocean search specialist. "Even when you know roughly where the target is, it can be very tricky to find the pinger. They have a very limited range."

A U.S. government official familiar with the investigation told CNN that based on present search patterns and available data, he believes it's far more likely that the plane would be located in the southern arc of the search area.

"This is an area out of normal shipping lanes, out of any commercial flight patterns, with few fishing boats and there are no islands," he said, warning that the search could well last "weeks and not days."