F-16 fighter planes scrambled to intercept an intruder in the sky. Residents on the ground were rattled by a loud boom. And then the private plane that caused so much alarm by veering into the airspace over Washington, D.C., suddenly crashed into dense woods in Virginia.
But the mystery of what caused the Cessna to go down Sunday, killing all four people aboard, remains.
So far, the National Transportation Safety Board, which is investigating the deadly crash, has said nothing about the likely cause.
“Everything is on the table until we slowly and methodically remove different components and elements that will be relevant for this safety investigation,” NTSB investigator Adam Gerhardt said after the crash.
That, experts and investigators who examine plane crashes have told NBC News, could point to hypoxia, when the brain is deprived of an adequate supply of oxygen, as a culprit.
"This has all the hallmarks of a pressurization issue of some kind," said Jeff Guzzetti, an aircraft accident investigator who has worked for both the NTSB and Federal Aviation Administration. "If this was a deliberate act, the plane would have been rammed into the ground or into a building and not allowed to fly until it ran out of fuel."
So investigators most likely are looking at whether the oxygen systems on the plane were properly maintained and whether the warning systems were working, Guzzetti said.
What causes hypoxia on flights?
Hypoxia, according to The Cleveland Clinic, is a condition in which there is a decrease in the supply of oxygen to body tissues. Chronic heart and lung conditions can put a person at risk for hypoxia. It can also be life-threatening.
"The most common causes of hypoxia in aviation are: flying, non-pressurized aircraft above 10,000 ft. without supplemental oxygen, rapid decompression during flight, pressurization system malfunction, or oxygen system malfunction," according to the Federal Aviation Administration.
Hypoxia symptoms include headaches, dizziness, sweating or shortness of breath, the FAA website says. Those symptoms are not always readily apparent and can creep up even on experienced pilots.
“One factor that makes hypoxia dangerous is its insidious onset; your signs and symptoms may develop so gradually that they are well established before you recognize them,” the FAA said. “Hypoxia is painless, and the signs and symptoms vary from person to person.”
What might have happened?
It's not clear what happened with the Cessna 560 Citation V, which is built to cruise at a maximum altitude of 51,000 feet and can carry seven to 12 passengers. That plane model is also equipped with an auto-pilot system, which is activated after takeoff and switched off before landing.
John M. Cox, a former US Airways captain and an NBC News contributor, said the pilot, Jeff Hefner, “stopped talking with the tower about 15 minutes after takeoff, at which point the plane was at about 30,000 feet and still climbing. “
“If this was case of decompression, you would have started losing cognitive ability within a short time,” Cox said. “You think you’re OK, but as the hypoxia sets in, even simple tasks, like adding numbers, become difficult or impossible. But assuming they were at 30,000 and climbing, they would have had about one to two minutes of useful consciousness before passing out.”
But he said this plane's autopilot system did what it was supposed to do.
"They were cleared to fly a specific route to Islip airport and they did," Cox said. "When they overflew the airport the autopilot system followed a compass heading as the pre-programmed route was complete. It followed that heading until it ran out of fuel and crashed."
What we know about the flight?
The Cessna took off Sunday from Elizabethton, Tennessee, at 1:13 p.m. ET before air traffic controllers radioed at 1:28 p.m. asking it to stop its climb at 33,000 feet, the senior government official said.
The plane, which at first was headed northeast toward Long Island, New York, changed course near New York City and began flying south when fighter jets from Joint Base Andrews, Maryland, were sent to investigate, causing a sonic boom as they flew over Washington.
The plane ran out of fuel and crashed near Montebello, Virginia, at 3:32 p.m.
For the final two hours of the flight, the pilot was silent and not responding to air traffic controllers, the government official said.
John Rumpel, whose corporation, Encore Motors of Melbourne, Florida, is the registered owner of the aircraft, identified three of the victims: Adina Azarian, 49; her daughter, Aria, 2; and pilot Hefner.
Rumpel did not divulge the name of the fourth person, who he said worked as a nanny taking care of the 2-year-old. He described Azarian as a former employee whom he and his wife, Barbara, adopted as a daughter when she was 40 years old.
Has hypoxia been suspected in other crashes?
Yes. In 1999, professional golfer Payne Stewart and five other people died when the Learjet they were aboard crashed. Federal investigators cited cabin depressurization and ultimately hypoxia as the cause of death.
Hypoxia is also one of the leading theories in the 2014 disappearance of Malaysia Airlines Flight 370, which vanished while flying over the South China Sea. The plane, and the 227 passengers and 12 crew aboard, has never been found.