(CNN) -- The pilot or pilots aboard an unresponsive plane that flew from the United States over the Caribbean Sea on Friday may have been stricken by cerebral hypoxia -- a fancy term for when your brain is deprived of oxygen.
The North American Aerospace Defense Command (NORAD) scrambled fighter jets to check on the plane and suggested on Twitter that hypoxia may be to blame.
Before the small plane's windows frosted, fighter pilots tracking the plane could see a pilot slumped over, a NORAD official said.
Cerebral hypoxia can happen to pilots if they reach too high of an altitude or if there is a loss in cabin pressure. It can also be the result of carbon monoxide poisoning or breathing in too much smoke from a fire.
There is a progressive reduction of oxygen per breath the higher the altitude.
Every pilot's reaction to hypoxia is different, according to a the Federal Aviation Administration. But is extremely hard for a person to tell when it is happening to them because the onset of symptoms is subtle.
In the beginning, pilots may experience an increase in the rate at which they breathe. They also may feel lightheaded or dizzy. Eventually, they will start to lose coordination. They will experience tunnel vision and their judgment would become impaired.
Brain cells are particularly vulnerable to a lack of oxygen. Without it they can die off and cause permanent brain damage.
When a brain goes without oxygen for too long, the part of the brain that helps with breathing can stop working and can keep you from breathing.
Federal investigators believe golfer Payne Stewart's Learjet went down in 1999 after his plane lost cabin pressure during its flight.
In that case, a low-pressure alarm could be heard on the cockpit voice recorder recovered from the wreckage. The FAA said his plane climbed as high as 51,000 feet. The human body has limited ability to function above 10,000 feet without additional oxygen.