As investigators continue to comb through the wreckage of a private plane that crashed Sunday in Virginia, the theory that is emerging is that the pilot and the three passengers on board the aircraft died of hypoxia.

CNN reported Tuesday that just 15 minutes after the plane took off from Elizabethton, Tennessee, en route to Long Island MacArthur Airport in New York, the flight control officials lost contact with the Cessna Citation jet.

The loss of communications occurred when the plane was at 31,000 feet.

The jet continued up to 34,000 feet and then flew past its destination of MacArthur Airport, then turned and started heading back south toward Washington, D.C.

The Cessna was believed to be on autopilot.

Six F-16 were launched from three bases to intercept the plane, White House National Security Council Coordinator for Strategic Communications John Kirby told reporters Monday.

John Kirby on Sunday's NORAD intercept: "Six F-16s from three different air bases...launched to intercept this particular Cessna Citation...the two from Andrews...they had to turn on the speed to get to him, which is why people here in the District area heard a sonic boom..." pic.twitter.com/0PKiqikPAk

— CSPAN (@cspan) June 5, 2023

The Washington Post, citing unnamed sources, said that an F-16 pilot told investigators that he saw the Cessna pilot, identified as Jeff Hefner, slumped over his seat in the cockpit.

The F-16 pilot encountered the Cessna about 12 minutes before it crashed in the mountains near Waynesboro, Virginia.

“Whatever happened, happened at altitude, which is a critical location to lose pressurization. The higher up in altitude you are, the less time you have to get on oxygen,” former Federal Aviation Administration and National Transportation Safety Board investigator Jeff Guzzetti told the Post.

Pilots have "30 to 60 seconds to don oxygen masks if the plane is depressurized or risk falling unconscious" due to hypoxia, according to CNN.

"The onset of symptoms is so subtle that it’s hard for a person to tell when it is happening to them. They might begin breathing at an increased rate, feel dizzy, lose coordination and experience impaired judgment," the news outlet said.

"When a brain goes without oxygen for too long, the part of the brain that helps with respiration can stop working and prevent a person from breathing."

Investigators are looking for the "black boxes" containing flight data, though the Cessna private plane was not required to have them.

Using information from the cockpit recorder, NTSB concluded a 1999 plane crash that killed professional golfer Payne Stewart was likely caused by hypoxia.

Guzzetti told The Post that the NTSB will be seeking to determine what caused the cabin to depressurize on the Cessna on Sunday and why Hefner was not able to put on his oxygen mask before going unconscious.

The plane was registered to Encore Motors of Melbourne, Florida.

In addition to Hefner, company owner John Rumpel said those who died on the airplane included his daughter Adina Azarian, his 2-year-old granddaughter, Aria, and the child's nanny.

Rumpel is a prominent contributor to Republican candidates and causes, including former President Donald Trump, Florida Gov. Ron DeSantis, Sen. Mitt Romney, and numerous state Republican parties, according to Open Secrets.

This article appeared originally on The Western Journal.