It was only 15 minutes after a Cessna 560 Citation civilian jet took off from an airport in northeastern Tennessee on Sunday that the aircraft, bound for New York, stopped responding to air traffic controllers, the Federal Aviation Administration said on Monday -- as new details emerged about an unusual episode in the skies over the nation's capital.
About 80 minutes after takeoff, the Cessna, having apparently been unresponsive to the outside world for more than an hour, made it to the intended destination on Long Island, flight tracking software shows.
But rather than landing at MacArthur Airport, the plane turned around.
At that point, one U.S. official told ABC News, concerns grew significantly. Until then, the Cessna had been following its flight path and was being tracked on radar by the FAA, even despite the lack of communication.
Around 3 p.m. ET, and nearly two hours after the Cessna left Tennessee and an hour and a half after last communicating with aviation officials, the plane, flying at an altitude of 34,000 feet, was drawing closer to the restricted airspace over Washington, D.C.
That's when six F-16s were scrambled by NORAD, or North American Aerospace Defense Command, to intercept it, a U.S. defense official said.
As the military jets raced toward the craft from three bases in Maryland, New Jersey and South Carolina, one of the F-16s from New Jersey is believed to have created a thunderous sonic boom heard over much of Washington and its suburbs, according to NORAD.
Around 3:20 p.m., two F-16s that had taken off from Joint Base Andrews in Maryland were the first to reach the aircraft, about 20 miles northeast of Reagan National Airport, according to the defense official.
The military pilots attempted to communicate with the unresponsive Cessna by radio and by rocking their wings, firing flares and flying around the aircraft, according to this official.
"NORAD pilots visually confirmed that the Cessna pilot was unresponsive," a second U.S. official said.
Around 3:32 p.m., according to the tracking software, the plane crashed in a remote, mountainous area near Montebello, Virginia. Authorities said the pilot and three passengers onboard were killed.
How the alarm was raised
The FAA, NORAD and other federal agencies had tracked the civilian jet as it headed northward after departing from the city airport in Elizabethton, Tennessee. Once the pilot failed to respond to air traffic control around 1:28 p.m., the FAA reported the issue about eight minutes later to a network that included "the military, national security, homeland security and other law enforcement agencies."
NORAD and other federal agencies were abreast of the situation on the conference call established for the incident.
But NORAD's view of the situation changed after the plane turned back over Long Island because it was now flying on an unknown flight plan which raised security concerns, one of the U.S. officials said.
NORAD can decide for itself if it will scramble jets based on its own criteria or after discussion with the other agencies. In such cases, military planes are deployed so they can provide "eyes on" an aircraft that is not responding to communications.
A U.S. official noted that occasionally pilots experience radio trouble or other issues that could cause a temporary loss of communication, leading flight controllers to attempt to reach them while monitoring the craft for any other odd activity.
"The actual standards [for scrambling jets] are classified, but there is a good bit of judgement involved," retired Marine Col. Stephen Ganyard, an ABC News contributor, said on "Good Morning America" on Monday.
It appears that military officials decided not to shoot the Cessna down because it was maintaining a constant high altitude and heading, meaning it wasn't seen as a threat, Ganyard said.
Both the FAA and the National Transportation Safety Board are investigating.
"We are here not only to figure out what happened but why this happened, to prevent future accidents," Adam Gerhardt, a NTSB senior air safety investigator, told reporters at a news conference near the crash site on Monday.
He said the area is extremely rural and mountainous and the "destroyed" wreckage will need to be lifted out by helicopter and brought to a secure facility in Delaware.
The NTSB has said the Cessna was not required to have a black box.
A preliminary report from NTSB investigators is expected within three weeks, officials said. That report will look at "the human, machine and environment as the outline of the investigation."
"At this early stage of an investigation, NTSB does not state a cause but will provide factual information when available," the board said.
The man who runs the company that owns the Cessna told The New York Times that the victims included his daughter, 2-year-old granddaughter and her nanny.
Adina Azarian, a prominent New York real estate agent and her daughter, Aria, were identified as among the dead, according to Azarian's employer.
"My family is gone, my daughter and granddaughter," Azarian's mother, Barbara Rumpel, posted on Facebook,
Experts said it was likely hypoxia caused the pilot and passengers to be unresponsive. Hypoxia is a condition that occurs when the body loses oxygen -- and severe hypoxia can cause a loss of consciousness and eventually death.
If an aircraft isn't pressurized, hypoxia can occur at or above 10,000 feet. At a higher altitude like 34,000 feet, hypoxia can occur much more rapidly, causing unconsciousness in around a minute to a minute and a half.
"The pressurization in the aircraft should keep enough oxygen in the cabin to stay alert and stay awake," Ganyard said in an interview on "World News Tonight."
But "[hypoxia] can happen insidiously," Ganyard said, "where you lose consciousness, you begin to feel tingling, you feel a sense of euphoria and it very slowly overcomes the people in the cabin."
ABC News' Clara McMichael, Lauren Minore and Kevin Shalvey contributed to this report.