In the cockpit of a private plane that was being pursued by fighter jets before coming down in Virginia, the pilot was reportedly found slumped over, according to US media.
After intercepting the plane, fighter jets spotted the pilot who appeared to be unconscious, according to reports by the Washington Post and CNN.
In the collision on Sunday, the pilot and three additional passengers perished.
Before crashing in Virginia, the Cessna Citation flew through restricted airspace over Washington, D.C.
When the aircraft arrived in New York after departing from Tennessee for Long Island, it made a sharp turn and then returned southward to its starting point.
It entered some of the most restricted airspace over the US capital, forcing F-16 fighter jets to fly at supersonic speeds to intercept it, which caused a loud sonic boom to reverberate throughout the area.
The aircraft ran out of fuel and crashed in a heavily forested, mountainous area close to Montebello in Virginia after the pilot remained silent for the final two hours of the flight.
The cause of the pilot's non-responsiveness is unclear. The plane was not shot down, according to military officials who spoke on the record under the condition of anonymity.
The wreckage is said to be "highly fragmented," and investigators are currently scouring rural Virginia for it. According to officials, that procedure will probably take days.
Adam Gerhardt, a National Transportation Safety Board investigator, stated that "everything is on the table" until he and his team gradually and methodically remove various parts and components that will be important to this safety investigation.
The following week, a report with more information will be released. Approximately 12 to 24 months after the fatal incident, a final report will be made public.
The identities of the four fatalities are still unknown, but a person connected to the plane claimed to have family members on board.
According to John Rumpel, 75, the owner of the Florida company that owned the plane, along with the pilot, the plane also carried his daughter, his granddaughter who was two years old, and her nanny.
He claimed that they had left his North Carolina home and were making their way back to East Hampton, New York.
Mr. Rumpel, a pilot himself, stated: "It descended at 20,000 feet per minute, and nobody could survive a crash from that speed." He also expressed his hope that his loved ones were spared.
A retired captain and pilot instructor named Richard Levy told BBC News that the Cessna likely lost cabin pressure.
According to him, a number of factors, such as pilot error or mechanical problems with the aircraft, can cause the cabin pressure to drop.
In this instance, Mr. Levy claimed that the cabin may have gradually and "insidiously" lost pressure without those on board ever noticing symptoms of hypoxia, a condition in which the body is deficient in oxygen, until it was too late.
They have passed the point of good judgment, consciousness, and vision because they are unaware of what is happening, according to Mr. Levy.
Mr. Levy speculated that the pilot may have attempted to turn the aircraft around while using the autopilot setting after realizing that the cabin was depressurizing. "I assume that the pilot then lost consciousness after that," he said.