US aviation officials on Monday were investigating the fatal crash of an "unresponsive" private plane that strayed over the nation's capital and prompted the scrambling of F-16 fighter jets.

The Cessna Citation slammed into mountainous terrain Sunday afternoon in Virginia, some 170 miles (275 kilometers) southwest of Washington, killing all four people aboard, officials said.

The crash came shortly after the North American Aerospace Defense Command (NORAD) dispatched F-16s to intercept an "unresponsive" Cessna 560 Citation V aircraft over Washington, whose airspace is tightly restricted.

According to The Washington Post, the pilot of one of the F-16s could see the pilot of the Cessna slumped over in the cockpit -- suggesting a loss of consciousness due to depressurization of the aircraft.

NORAD said flares were deployed to try to draw the attention of the pilot but there was no response and the private plane eventually crashed near the George Washington National Forest in Virginia.

"NORAD attempted to establish contact with the pilot until the aircraft crashed," it said in a statement.

The F-16s triggered a sonic boom across Washington and its suburbs, startling residents and rattling windows for miles.

"The NORAD aircraft were authorized to travel at supersonic speeds and a sonic boom may have been heard by residents of the region," NORAD said.

Aviation experts speculated that the pilot of the Cessna may have become incapacitated due to a depressurization of the aircraft, which can cause a rapid loss of consciousness at altitudes above 10,000 feet (3,000 meters).

A loss of cabin pressure was blamed for a high-profile 1999 Learjet accident that killed golfer Payne Stewart and four other people.

In that case, the Learjet, which was on a flight from Florida to Texas, flew for thousands of miles on autopilot before eventually running out of fuel and crashing in South Dakota.

The Federal Aviation Administration (FAA) and National Transportation Safety Board said they were investigating Sunday's accident.

Virginia State Police said first responders reached the crash site near the town of Staunton by foot some four hours after the Cessna plummeted to the ground.

"No survivors were located," police said in a statement.

The Cessna had taken off from Elizabethton, Tennessee, bound for Long Island MacArthur Airport in New York, the FAA said.

But it turned around after flying over Long Island and headed back south over Washington and into Virginia, climbing as high as 34,000 feet according to flight tracking website Flightradar24.

The Post said contact with the plane was lost about 15 minutes after its departure from Elizabethton and the aircraft may have continued to fly for hours on autopilot before exhausting its fuel and crashing.

- 'My family is gone' -


President Joe Biden, who was at the White House and also played golf Sunday, was briefed on the incident, an official said without specifying whether any emergency precautions were implemented.

US authorities have yet to officially identify those on board, but comments by two relatives of people believed to have been on the plane provided some information.

Public records showed the aircraft was registered to Florida-based company Encore Motors of Melbourne, whose owner John Rumpel told the Post his daughter, a grandchild and her nanny were on board.

In response to condolence messages on her Facebook page, Rumpel's wife, Barbara, wrote on the platform Sunday night: "My family is gone, my daughter and granddaughter."