CNN aviation analyst Miles O'Brien called the fresh details about the flight a "game changer."
"Now we have no evidence the crew did anything wrong," he said. "And in fact, now, we should be operating with the primary assumption being that something bad happened to that plane shortly after they said good night."
If a crisis on board caused the plane to lose pressure, he said, pilots could have chosen to deliberately fly lower to save passengers onboard.
"You want to get down to 10,000 feet, because that is when you don't have to worry about pressurization. You have enough air in the atmosphere naturally to keep everybody alive," he said. "So part of the procedure for a rapid decompression ... it's called a high dive, and you go as quickly as you can down that to that altitude."
Military radar tracked the flight between 1:19 a.m. and 2:40 a.m. the day it went missing, the source told CNN, but it's not clear how long it took the plane to descend to 12,000 feet.
The new details about altitude are "highly significant," said Mary Schiavo, a CNN aviation analyst and former inspector general for the U.S. Department of Transportation.
"It explains so many pieces that didn't fit together before," she said. "Now, if we have a scenario where something happened, the plane made a dramatic turn and dropped from 35,000 feet to 12,000 feet, this scenario would fit what a pilot would do in the event of a catastrophic onboard event, such as a rapid decompression, a fire, an explosion. That's what you would have to do, descend, get down and turn around and try to get back to an airport that could accommodate an ailing plane."
If the latest information is accurate, the theory of pilots trying to save the plane fits, said Mark Weiss, a former American Airlines pilot and CNN aviation analyst.
But that's a big if, he said.
"We've had so much information come out and so much contradictory information come out, that I caution against jumping to any types of conclusions at this point," he said.
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