By Corky Siemaszko - NBC News
F-16 fighter jets are known to have been sent to intercept an aircraft that violated Washington, D.C., airspace on Sunday, sowing panic among residents who felt a sonic boom. It is also known that the private plane eventually crashed in a dense wooded area of Virginia.
However, it is still a mystery that caused the Cessna plane to go down on Sunday and claim the lives of its four crew members.
So far, the National Transportation Safety Agency (NTSB), which is investigating the deadly crash, has said nothing about the cause.
Members of the Virginia State Police arrive at the site where a plane crashed in a wooded area on Sunday. Getty Images
"Everything is on the table until, slowly and methodically, different components and elements that may be relevant to this investigation are analyzed," Adam Gerhardt, an NTBS investigator said after the accident.
However, a senior official told NBC News that the pilots of the fighters who chased the plane saw the Cessna pilot lying on the controls, unconscious.
Experts and researchers studying plane crashes told NBC News that the pilot's condition was likely caused by a disorder called hypoxia, which occurs when the brain is deprived of the necessary amount of oxygen.
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"This has every sign of some kind of problem with pressure," said Jeff Guzzetti, an air accident investigator who has worked for the NTSB and the Federal Aviation Administration. "If it was a deliberate act, I would have crashed the plane into the earth or a building."
So investigators are looking into whether the plane's oxygen systems had been properly maintained and whether the alarm signals were working, Guzzetti said.
What causes hypoxia?
Hypoxia, according to the Cleveland Clinic, is a condition in which there is a reduction in oxygen supply to body tissues. Chronic heart and lung conditions can put a person at risk for hypoxia, which can be life-threatening.
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"The most frequent causes of hypoxia in aviation are: flying an aircraft that is not pressurized above 10,000 feet without an additional oxygen supply, rapid decompression during flight, and pressurization system or oxygen system failures," according to the Federal Aviation Administration (FAA).
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Symptoms of hypoxia are usually headaches, dizziness, sweating or shortness of breath, the FAA describes on its website. The symptoms are not immediately visible and can surprise even experienced pilots.
"One factor that makes hypoxia so dangerous is its insidious onset. Signs and symptoms can develop gradually, to the point of being well established when they can be recognized," the FAA said. "Hypoxia is not painful, and the signs and symptoms vary from person to person."
What could have happened?
It's unclear what may have happened to the Cessna 560 Citation V, which was built to fly at a maximum altitude of 51,000 feet and carry 12 passengers. That model of aircraft is also equipped with an autopilot system, which activates after takeoff and is turned off before landing.
John M. Cox, a former US Airways captain and NBC News contributor, said the pilot, Jeff Hefner, "stopped communicating with the tower about 15 minutes after takeoff, at a time when the plane was at 30,000 feet and continued to gain altitude."
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"If it was a case of decompression, he started losing cognitive abilities in a short time," Cox said. "You think you're fine, but hypoxia is happening. Even simple tasks, such as adding numbers, become difficult or impossible. But assuming they were at 30,000 feet and rising, they had a minute or two before they lost consciousness."
He said the plane's autopilot did what it was supposed to do.
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"They received authorization to fly a specific route to Islip Airport and they did," he said. "When they passed over the airport, the autopilot continued in the direction marked on the compass once the scheduled route was completed. It continued in that direction until the fuel ran out and the ship crashed."
What do we know about the flight?
The Cessna plane departed Sunday from Elizabethton, Tennessee, at 1:13 p.m. Then, at 1:28 p.m., flight controllers told him to stop ascending to 33,000 feet, a government official said.
The plane, which was initially heading northeast toward Long Island, New York, changed course near New York and began flying south. At that time F-16s from Andrews Air Force Base in Maryland were sent to investigate, which caused the loud sound over Washington.
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The plane ran out of fuel and crashed near Montebello, Virginia, at 3:32 p.m.
For the last two hours of the flight, the pilot was quiet and did not respond to air traffic controllers, government officials said.
John Rumpel, whose corporation, Encore Motors of Melbourne in Florida, is the registered owner of the aircraft, identified three of the victims as Adina Azarian, 49, her daughter, Aria, 2, and pilot Hefner.
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Rumpel did not name the fourth victim, who said she worked as the girl's nanny. He described Azarian as a former employee whom he and his wife, Barbara, had taken in as a daughter when she was 40.
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Has hypoxia caused other accidents?
Yes. In 1999, professional golfer Payne Stewart and five others died when the Learjet plane they were traveling in crashed. Federal investigators pointed to cabin decompression and hypoxia as the cause of the death.
Hypoxia is also one of the theories for the disappearance of Malaysia Airlines Flight 370 in 2014, which vanished while flying over the South China Sea. The plane, which was carrying 227 passengers and 12 crew, has not been found.
