Mystery Virginia plane crash debris 'highly fragmented'

US investigators have begun combing through rural Virginia in search of wreckage from a small plane that flew off course before crashing on Sunday.

The Federal Aviation Administration (FAA) said on Monday the pilot and three died in the incident, but did not disclose their identities.

It will take days to assess the "highly fragmented" wreckage, scattered in a mountainous region, they said.

A person linked to the small plane's owner said family members were onboard.

John Rumpel, 75, who runs the Florida business that owned the plane, told the New York Times his daughter, two-year-old granddaughter and her nanny were on the plane along with the pilot.

He said they had been returning to East Hampton, New York state, from his North Carolina home.

"It descended at 20,000ft a minute, and nobody could survive a crash from that speed," Mr Rumpel, who is also a pilot, said, adding that he hoped those on board had not suffered.

A report with further details will be released next week.

"Everything is on the table until we slowly and methodically remove different components and elements that will be relevant for this safety investigation," National Transportation Safety Board investigator Adam Gerhardt said.

Investigators will analyse the most delicate evidence while on the scene before the wreckage will be moved to Delaware for further inspection, Mr Gerhardt said. They will look at when the pilot first became unresponsive and what caused the Cessna 560 Citation V private plane to take the path it did.

A final report on the fatal incident will be released in 12 to 24 months, he said.

The crash has drawn heightened interest because of its journey over Washington, DC and some of the country's most-restricted airspace.

The North American Aerospace Defense Command (Norad) said in a statement that US F-16 fighter jets were sent out at supersonic speeds in order to intercept the plane, causing a loud sonic boom to be heard in the DC region.

"During this event, the Norad aircraft also used flares - which may have been visible to the public - in an attempt to draw attention from the pilot," the statement said.

A sonic boom is similar to thunder, the result of an object moving faster than sound - about 750mph (1,207km/h) at sea level - according to the US Air Force. The sound can shatter glass, but is usually harmless to those that hear it.

The Cessna then crashed in a rural area of Virginia after a rapid, spiralling descent. Military officials speaking on condition of anonymity said it was not shot down and the fighter jets did not cause the crash.

Police and rescuers reached the wreckage near the George Washington National Forest by foot hours after the crash and said no survivors had been found.

Richard Levy, a retired captain and pilot instructor, told BBC News the Cessna probably lost cabin pressure.

Aircraft cabins can depressurise for a number of reasons, including because of aircraft mechanical malfunctions or pilot errors, he said.

In this case, Mr Levy said the cabin may have depressurised gradually and "insidiously" without those on board even noticing symptoms of hypoxia - a condition in which the body is deprived of adequate levels of oxygen - until it was too late.

"They're unaware of what's happening, and then they've gone beyond the point of rational thinking, consciousness and good vision," Mr Levy said.

US aviation officials said the plane took off from Elizabethton, Tennessee, at 13:14 local time (18:14 BST) on Sunday and was heading to Long Island in New York.

But flight-tracking data shows the pilot made a hairpin turn after reaching Long Island before beginning a steady route back towards its origin, flying over the protected airspace around the US capital.

The flight data ends at around 15:30 local time near Montebello in Virginia and shows the aircraft descended at great speed.

Mr Levy said the pilot may have realised at one point that the cabin was depressurising and then tried to turn the aircraft around on an autopilot setting.

"After that, my assumption is that the pilot then lost consciousness," he said.

US President Joe Biden was briefed on the incident at the time, officials said.

He was playing golf at Joint Base Andrews in Maryland where the fighter jets departed from.

The Cessna crash has drawn comparisons with the 1999 crash of a Learjet carrying professional golfer Payne Stewart and five other passengers.

After losing pressure, that plane flew directionless for thousands of miles across the country before crashing in South Dakota, killing everyone on board.