The pilot of a military jet that was scrambled to intercept a private plane that flew over D.C. before crashing in rural Virginia saw that aircraft’s pilot slumped over, according to two people familiar with the situation.

The development, described by people speaking on the condition of anonymity because of the ongoing investigation, came as National Transportation Safety Board investigators on Monday arrived at the scene of a private jet crash, which was linked to a sonic boom heard a day earlier across the Washington region as F-16s flew to intercept the aircraft.

Adam Gerhardt, the lead investigator, said that he expects his team to be on the scene for three or four days and that investigators will be dealing with remote, mountainous terrain.

“The wreckage is highly fragmented,” Gerhardt said near the scene Monday.

Experts said publicly available flight data suggests that the pilot had fallen unconscious — probably because of a loss of pressurization — and that the plane was on autopilot until it ran out of fuel. The Federal Aviation Administration said preliminary information shows the pilot and all three passengers died, which it said occurred “under unknown circumstances.”

The plane that crashed was registered to Encore Motors of Melbourne, a Florida-based company owned by John Rumpel. He said the plane departed Elizabethton, Tenn., to carry his daughter, Adina Azarian; 2-year-old granddaughter, Aria; and their nanny to their home in East Hampton, N.Y. Rumpel identified the pilot as Jeff Hefner, who he said had been flying his planes “on and off” for more than five years. The nanny’s name was not available Monday.

FAA records show Hefner was an experienced pilot with a rating that qualified him to fly Boeing 737s, among other jets. He had a top-level medical certificate as recently as October.

Rumpel said authorities told him the plane might have crashed after losing pressure, which could have caused those inside to lose consciousness.

The Cessna Citation departed Sunday from Tennessee and was bound for Long Island, but it turned back south after reaching New York and eventually flew over D.C. The NTSB said contact with the plane was lost during its ascent, at about 31,000 feet, about 15 minutes after departure as it was passing over Virginia the first time.

The two people familiar with the investigation said the pilot of a military jet saw the Citation’s pilot sitting in the left seat and slumped toward the right.

The plane continued to fly at about 34,000 feet — overflying Long Island MacArthur Airport — until it begin to descend at 3:23 p.m., spiraling to the ground and crashing nine minutes later.

Six F-16s were scrambled from Joint Base Andrews in Maryland and two other facilities, a Pentagon spokesman said. The F-16s were granted rare authorization by commanders to fly at supersonic speeds over an urban area for the intercept, causing the boom and indicating the urgency of the military’s response.

“It is important for the responding aircraft, in this case F-16s, to reach the situation as quickly as possible,” said Lt. Col. Devin Robinson, a Pentagon spokesman. “This allows more time for the people on the ground to run through procedures and gives more time for decisions to be made.”

The military jets and air traffic controllers were unable to make contact with the plane, officials said, and it crashed about 3:30 p.m. in Augusta County, more than a mile from the Blue Ridge Parkway. Virginia State Police said remains recovered at the scene will be taken to the Office of the Virginia Medical Examiner for examination and positive identification.

First responders reached the crash site about 8 p.m. Sunday, police said, and investigators hiked through the heavily wooded terrain. The wreckage will be moved to a facility in Delaware.

National Security Council spokesman John Kirby told reporters Monday that officials were on the phone and receiving real-time updates from the F-16 pilots, and that President Biden was kept informed.

“They did exactly what they were supposed to do,” Kirby said of the military pilots. “Try to get on the radio, communicate to the pilot. That wasn’t working. Made themselves visible. That didn’t work. And tragically, it ended, obviously, in the crash and the death of all on board.”

Recordings of radio communications included attempts by several aircraft and the FAA to reach the plane. Among those making attempts were pilots from an American Airlines flight, according to people familiar with the transmissions who were not authorized to speak publicly.

NTSB investigators will document the crash scene and examine the wreckage, as well as gather information from radar, weather data, the plane’s maintenance records and pilot medical records.

The board is expected to release a preliminary report in about three weeks, summarizing facts that investigators have gathered. A final report including a formal cause will probably take at least a year.

The plane made no attempt to descend on Long Island and appeared to have turned around to head back to Tennessee.

Hassan Shahidi, president and CEO of the nonprofit Flight Safety Foundation, said the route was perplexing and will be a focus for investigators. He said it would not be normal for a plane to set out for a destination, then make a U-turn and head toward where it started, even if autopilot capabilities were guiding the plane.

“It’s very unclear why it made this maneuver so close to its intended destination,” Shahidi said. “The aircraft, especially the Cessna airplane, it has to be directed by the pilot to take a certain flight path and altitude.”

Investigators will try to scrutinize all the different “maneuvering instructions” the aircraft received during its flight, from pilot or autopilot, including how high to fly and in which direction, he said. They will then compare those details with the timing of air traffic control communications and when the pilot stopped responding.

Jeff Guzzetti, a former FAA and NTSB investigator, said flight-tracking data suggests that long before the plane reached New York, the pilot was not in control.

“Whatever happened, happened at altitude, which is a critical location to lose pressurization,” Guzzetti said. “The higher up in altitude you are, the less time you have to get on oxygen.”

The plane continued to fly at about 34,000 feet until it began to spiral toward the ground. Guzzetti said the final minutes of the flight indicate fuel for the plane’s right engine was exhausted.

It will be up to the NTSB to determine what might have caused the plane to lose pressure and why the pilot was not able to use an oxygen system. Guzzetti said investigators will want to know when the oxygen system was last serviced and whether maintenance records reveal any issues with the plane.

“It’s going to be challenging for the NTSB to answer these questions given the destruction of the wreckage,” he said.

A spokesman for the North American Aerospace Defense Command said two jets “inspected” the Cessna, which was intercepted about 3:20 p.m. The pilot was unresponsive and the plane crashed near the George Washington National Forest in Virginia, officials said. The military did not shoot down the plane and there is no indication the interception caused the crash.

The crash location is about 160 miles southwest of Washington.

Records from the FAA indicate the plane was only recently acquired by Encore. Guzzetti said that transaction would have involved an inspection of the registration and ownership records by the FAA, but only a cursory one that probably would not have delved into the condition of the plane’s pressurization system.

The sonic boom, heard from Springfield, Va., to Bowie, Md., according to social media reports, startled residents in the Washington region Sunday afternoon. The source of the sound was a mystery for about an hour, until local authorities confirmed it had been caused by fighter jets.

The six F-16s were scrambled simultaneously in response to the situation, and included two with the 113th Fighter Wing from the D.C. Air National Guard reaching the Cessna first after departing from Joint Base Andrews in Maryland. The other jets were with the New Jersey Air National Guard’s 177th Fighter Wing, flying from Atlantic City, and the South Carolina Air National Guard’s Fighter Wing, responding from McEntire Joint National Guard Base, Robinson said.

Robinson said the intercept occurred about 20 miles northeast of Reagan National Airport. It was not clear Monday where the military plane crossed the sound barrier, a phenomenon that creates an air pressure change resulting in a deep boom.

The military often conducts intercepts of unidentified objects or aircraft that are not making customary check-ins with controllers on the ground, but it is unusual for commanders to authorize supersonic speed over an urban area.

Other recent intercepts by the U.S. military include scrambles earlier this year to reach unidentified objects flying over the United States, after an alleged Chinese surveillance airship made a cross-country flight beginning over Alaska and concluding with it being shot down by F-22s off South Carolina. Other objects were intercepted over Alaska and over the Midwest, with one shot down over Lake Huron.

Aviation industry experts, without reaching conclusions on the crash’s cause, on Monday pointed to the 1999 death of professional golfer Payne Stewart and five others after their business jet flew for hours without a pilot controlling the aircraft.

F-16 pilots who had scrambled to monitor the Learjet eventually watched as it dropped toward Aberdeen, S.D. Investigators from the NTSB concluded the pilots became incapacitated because they didn’t receive supplemental oxygen after the cabin lost pressure.

But in a sign of the complexities of such probes, investigators weren’t able to determine why the pilots couldn’t get the oxygen quickly enough.

Scott Wagner, an assistant professor in the Department of Aeronautical Science at Embry-Riddle Aeronautical University, said incidents involving the loss of pressurization on an aircraft are uncommon.

A high-profile instance occurred in 2018 after a Dallas-bound Southwest Airlines flight was forced to make an emergency landing at Philadelphia International Airport after one of its engines exploded and debris shattered a window, causing rapid loss of cabin pressure.

Wagner said in cases involving a loss of cabin pressure, the crew and pilots might have only a minute to put on masks before they begin to feel the effects — symptoms that can include confusion, numbness or a tingling feeling in fingers and toes.

According to the NTSB, hypoxia — a lack of sufficient oxygen in the blood and body tissue — can cause drowsiness and slurred speech, loss of consciousness or death. The safety agency cited research showing that the rapid depressurization of an airplane cabin can impair the cognitive function of pilots in eight seconds.