WASHINGTON (AP) — The pilot of a business jet that flew over Washington and crashed into a remote part of Virginia on Sunday appeared to be slumped over and unresponsive, fighter jet pilots reported, according to two U.S. officials briefed on the matter.

Federal investigators trudged through rugged terrain Monday in search of wreckage from a business jet to solve the mystery of why the plane veered off course and slammed into a mountain, killing four people.

A day after the plane flew over the nation’s capital, prompting the military to scramble fighter jets, the Federal Aviation Administration said in a brief update that the pilot and three passengers were killed and that the plane was “destroyed” in the crash. Their identities weren’t immediately released.

The pilot of the business jet that flew over Washington and crashed in Virginia appeared to be slumped over and unresponsive before the crash, the fighter jet pilots reported, according to two U.S. officials briefed on the matter. The officials were not authorized to discuss details of the military operation and spoke on condition of anonymity.

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NTSB investigator Adam Gerhardt told reporters it will take investigators a while to reach the remote crash scene about two to three miles north of Montebello. They expect to be on the scene for at least three to four days. Investigators had to hike to the site on foot because of the mountainous terrain.

Attention on the crash and its cause was heightened by its unusual flight path over Washington, D.C. and a sonic boom caused by military aircraft heard across the capital, and parts of Maryland and Virginia. The North American Aerospace Defense Command said in a statement that military aircraft was authorized to travel at supersonic speeds, which caused the sonic boom. The aircraft also used flares to try to get the pilot’s attention.

Air Traffic Control audio from the half-hour before the plane crashed captures voices identifying themselves as military pilots trying to communicate with the pilot of the plane, according to recordings on LiveATC.net.

“If you hear this transmission, contact us,” said one pilot who identifies as being with the Air National Guard.

Several minutes later, a military pilot says: “You have been intercepted. Contact me.”

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Speaking at a briefing Monday morning, Gerhardt said the wreckage is “highly fragmented” and investigators will examine the most delicate evidence on the scene, after which the wreckage will be moved, perhaps by helicopter, to Delaware, where it can be further examined, he said. The plane is not required to have a flight recorder but it is possible that there are other avionics equipment that will have data that they can examine, Gerhardt said.

Virginia State Police said the crash site is more than a mile (1.6 kilometers) from the Blue Ridge Parkway in Augusta County, near the Nelson County line. Along with NTSB, they are beginning evidence collection and body recovery efforts. Remains collected at the scene will be transported to the medical examiner’s office in Virginia for autopsy and positive identification.

Investigators will look at when the pilot became unresponsive and why aircraft flew the path that it did, he said. They will consider several factors that are routinely examined in such probes including the plane, its engines, weather conditions, pilot qualifications and maintenance records, he said.

“Everything is on the table until we slowly and methodically remove different components and elements that will be relevant for this safety investigation,” Gerhardt said.

A preliminary report will be released in 10 days and a final report will be released in one to two years, he said.

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Meanwhile, the White House expressed its “deepest condolences” on Monday to the family of those on board the plane.

“We need to keep them front and center,” National Security Council spokesperson John Kirby said.

Kirby deferred questions about a follow-up report on the security response over Washington airspace to the Pentagon and U.S. Secret Service. But he said, “What I saw was just a classic, textbook response.”

The White House was continuously informed as Air Force jets tried to contact the pilot of the civilian plane and monitored the small aircraft’s path from Washington airspace to rural Virginia, Kirby said.

Police said Sunday night that rescuers had reached the crash site in a rural part of the Shenandoah Valley and that no survivors were found. Virginia State Police said officers were notified of the potential crash shortly before 4 p.m. and rescuers reached the crash site by foot around four hours later.

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The FAA said the Cessna Citation took off from Elizabethton, Tennessee, on Sunday and was headed for Long Island’s MacArthur Airport. Inexplicably, the plane turned around over New York’s Long Island and flew a straight path down over D.C. before it crashed around 3:30 p.m.

The plane flew directly over the nation’s capital, though it was technically flying above one of the most heavily restricted airspaces in the nation.

According to the Pentagon, six F-16 fighter jets were immediately deployed to intercept the plane. Two aircraft from the 113th Fighter Wing, out of Joint Base Andrews in Maryland, were the first to reach the Cessna to begin attempts to contact the pilot. Two F-16 aircraft out of New Jersey and two from South Carolina also responded.

Flight tracking sites showed the plane suffered a rapid spiraling descent, dropping at one point at a rate of more than 30,000 feet (9,144 meters) per minute before crashing in the St. Mary’s Wilderness.

In Fairfax, Virginia, Travis Thornton was settled on a couch next to his wife, Hannah, and had just begun recording himself playing guitar and harmonica when they were startled by a loud rumble and rattling that can be heard on the video. The couple jumped up to investigate. Thornton tweeted that they checked in with their kids upstairs and then he went outside to check the house and talk to neighbors.

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The plane that crashed was registered to Encore Motors of Melbourne Inc, which is based in Florida. John Rumpel, a pilot who runs the company, told The New York Times that his daughter, 2-year-old granddaughter, her nanny and the pilot were aboard the plane. They were returning to their home in East Hampton, on Long Island, after visiting his house in North Carolina, he said.

Rumpel told the newspaper he didn’t have much information from authorities but suggested the plane could have lost pressurization.

“It descended at 20,000 feet a minute, and nobody could survive a crash from that speed,” Rumpel told the newspaper.

The episode brought back memories of the 1999 crash of a Learjet that lost cabin pressure and flew aimlessly across the country with professional golfer Payne Stewart aboard. The jet crashed in a South Dakota pasture and six people died.