While the search for Malaysia Airlines Flight 370 resumes in the southern Indian Ocean, government authorities and airline officials are telling the world, "All lives are lost."

Citing complicated math and satellite communications technology, Malaysian officials are even saying they know how and when Flight 370 ended. It crashed into the Indian Ocean, on March 8 between 8:11 and 9:15 a.m. local time, they say.

But what still remains a mystery, vexing investigators and gripping the world, is why.

The uncertainty is overwhelming for relatives of the 239 people aboard the plane, some of whom even marched to the Malaysian Embassy in Beijing to denounce the airline, the country and just about everything involved with the investigation.

As authorities in Malaysia and across the world are working around the clock to piece together just what happened, two primary theories are emerging - each with their own supporting evidence.

CNN's Barbara Starr on the evidence it may have been deliberate action.

U.S. officials confirm that a deliberate act has still not been ruled out. Whoever was at the controls appears to have deliberately abandoned the original flight path for Beijing and made a deliberate left-hand turn just before Vietnam, sources tell CNN's Barbara Starr.

There has been no claim of responsibility by any terrorist organizations, and there was never a mayday warning from the cockpit of a hijacking.

Still, if there had been an accident or technical failure, why didn't the pilots reach out?

"If you had an emergency, if you had an explosive decompression, if you had a fire... you want to get that airplane on the ground as quickly as possible at the nearest and suitable airport at the nearest point in time," says CNN Aviation Analyst Mark Weiss.

CNN's Joe Johns on the evidence it may have been a deadly accident.

Other experts say a deliberate act doesn't add up and the evidence really points to an accident.

According to this theory, sometime after the famous "All right, good night" transmission, a "catastrophic decompression" of the plane takes place, caused by anything from a partially open door to sudden smoke or fire.

As the pilots alter their course and seek safe harbor, they are overwhelmed by the lack of oxygen. There may never have been time for a mayday call, experts tell CNN’s Joe Johns.

"I think in the first few minutes, the pilots had to change course for an emergency airport. They were overtaken by whatever it was - smoke, fire. The plane was left to fly itself after being reprogrammed," says Clive Irving, a contributor to The Daily Beast and senior consulting editor at Condé Nast Traveler.

This would also explain the changes in altitude, as the pilots tried to return oxygen to the cabin, and why the plane flew in a straight course over the Indian Ocean, as the plane - with no one to guide it - remained on autopilot and flew until it ran out of fuel.

Whatever the answer is, the world may never know. Without any debris or the all-important flight data recorder, theories about what happened aboard Flight 370 are just that.

Authorities caution that despite narrowing the search area, it could still be some time before crews find any sign of the plane.