The pilot of a business jet that flew over Washington and crashed in a remote part of Virginia appeared to be slumped over and unresponsive, two U.S. officials said Monday, recounting observations by fighter pilots who intercepted the wayward flight.

The revelations came as federal investigators trudged through rugged terrain to reach the site where the plane slammed into a mountain Sunday, killing four people. The New York Times reported that the daughter and 2-year-old granddaughter of the company’s owner were aboard the plane. The officials who said that the fighter pilots saw the civilian pilot slumped over had been briefed on the matter and spoke on condition of anonymity because they were not authorized to discuss details of the military operation.

On Sunday, the private plane’s erratic flight path — inexplicably, turning around over New York’s Long Island to fly directly over the nation’s capital — had prompted the military to scramble fighter jets, which caused a sonic boom heard in Washington, Maryland and Virginia.

Remote terrain around the crash site posed major challenges to the investigation. Eric Weiss, a spokesperson for the NTSB, said Monday afternoon that it was taking investigators several hours to hike into the rural area near the community of Montebello, about 60 miles (97 km) southwest of Charlottesville. They expect to be on the scene for at least three to four days

Speaking at a briefing Monday morning, NTSB investigator Adam Gerhardt said the wreckage is “highly fragmented” and investigators will examine the most delicate evidence at the site, after which the wreckage will be moved, perhaps by helicopter, to Delaware, where it can be further examined. 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.

The Virginia State Police issued a statement saying that because of the severity of the crash, human remains will be transported to the state medical examiner’s office for autopsy and identification. The Federal Aviation Administration said that the victims included the pilot and three passengers but didn’t release their names. There were no survivors.

Investigators will look at when the pilot became unresponsive and why aircraft flew the path that it did, Gerhardt 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. A preliminary report will be released in 10 days.

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 the military 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.

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

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

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

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

The plane flew directly over the nation’s capital. 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.

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 said his family was returning to their home in East Hampton, on Long Island, after visiting his house in North Carolina.

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.