The recovery of the private plane that crashed in Virginia, killing all four people onboard, after its pilot became unresponsive and veered dramatically off course, is expected to begin Tuesday as investigators face challenging rural terrain and the near total devastation of the aircraft, a federal official said.

Investigators with the National Transportation Safety Board spent Monday sifting through the wreckage as they try to determine when the pilot became unresponsive and what caused the aircraft to fly hundreds of miles off course, eventually spurring the deployment of six fighter jets as it flew near Washington, DC, officials said.

The aircraft ultimately slammed into a heavily wooded area near Waynesboro, Virginia, and its impact left few recognizable pieces behind, NTSB investigator in charge Adam Gerhardt said Monday.

"The wreckage is destroyed, meaning that it is no longer distinguishable as an aircraft. However, there's still several pieces that might be able to assist our fact-finding stage at this point," Gerhardt said.

The agency plans to begin recovering salvageable pieces of the plane, a Cessna 560 Citation V, on Tuesday, he said.

The crash site is a lengthy hike through the densely forested, mountainous terrain, the NTSB told CNN. Gerhardt said the agency plans to helicopter pieces of the wreckage to a secure facility in Delaware in the coming days.

Investigators are scouring the crash scene for a flight data recorder or cockpit voice recorder, known as "black boxes," though the aircraft would not have been required to have them, Gerhardt said.

No survivors were found when the grisly crash site was discovered Sunday night, officials said, though signs of human remains were found, according to first responders who spoke on the condition that they not be identified.

A member of the family whose company, Encore Motors, owned the plane told the Washington Post that his daughter, her toddler and their nanny were among those killed in the crash.

John Rumpel, who confirmed to CNN that he and his wife Barbara own the company, told the Post that his daughter Adina Azarian, his 2-year-old granddaughter, Aria Azarian, and the nanny were supposed to be flying to their home in East Hampton, New York.

Rumpel told the Post that he got a call from the FAA about 90 minutes after dropping them off at an airport in eastern Tennessee. The agency asked if he knew how to contact the plane, he said.

Rumpel identified the pilot as Jeff Hefner, according to the report.

The private plane took off from the airport in Elizabethton, Tennessee, and overshot it's destination -- New York's Long Island MacArthur Airport -- before turning around and heading in the direction of Washington, DC, according to NORAD and LiveATC.net.

As it approached the capital region at a 34,000-foot altitude, air traffic controllers, civilian pilots and the F-16 pilots urgently tried to contact the unresponsive Cessna by radio, audio from LiveATC.net revealed.

Six F-16s were launched from three bases and raced to intercept the private plane, White House National Security Council Coordinator for Strategic Communications John Kirby said Monday.

The pilot was seen slumped over in his seat in the cockpit, a source familiar with the response told CNN.

A US official said the F-16s did not shoot down the aircraft. The official noted it is typical for the FAA to call in jets if someone is flying unsafely.

Source: Hypoxia being investigated as potential factor

Investigators are considering whether hypoxia -- a potentially deadly condition caused by lack of oxygen -- is a reason the pilot and passengers didn't respond to efforts to contact the plane, a source said.

Hypoxia poses a dangerous threat when flying at high altitudes and could have been caused if the Cessna's pressurized cabin suddenly depressurized, according to aviation experts.

At the 34,000-foot altitude the plane was at, pilots have 30 to 60 seconds to don oxygen masks if the plane is depressurized or risk falling unconscious.

Another key factor investigators are examining is the plane's autopilot function, a source familiar with the investigation said.

Deceased mother and pilot remembered as dedicated colleagues and parents

One of the passengers who was killed in the crash, identified by her father as Adina Azarian, was "a very devoted mother of her two-year-old daughter, Aria," her employer, real estate company Keller Williams, said in an online post.

Azarian began working for Keller Williams in 2011 after founding her own real estate firm, which the company said was "one of the first female-run real estate brands in NYC."

"Adina was known for her dedication, professionalism, and warm spirit. Her vibrant personality and unwavering commitment to her clients set her apart in the real estate industry," Keller Williams' New York City branch said in the post.

The nanny who Rumpel said was also on the plane has not been identified.

Hefner, the pilot identified by the Azarian's father, had flown numerous private aircraft after working as a commercial pilot, according to the head of a law firm he previously worked at as a flight captain.

Hefner was "a highly accomplished and skilled aviator," said attorney Dan Newlin. "He flew 25 years with as a captain with Southwest Airlines and had over 25,000 flight hours."

Hefner is survived by his wife and three children, Newlin said.

F-16s caused sonic boom rushing to intercept plane

Just 15 minutes after the plane took off in Tennessee, the FAA lost contact with the small jet, according to a statement from the agency and data from air travel tracking website FlightAware.

Shortly after the plane went silent, the agency alerted the "Domestic Events Network" that includes the military, national security, homeland security and other law enforcement agencies, according to the FAA statement.

It's unclear whether the plane entered restricted airspace.

As the Cessna continued to be unresponsive and flew near Washington, DC, the F-16 jets were "authorized to travel at supersonic speeds" in order to make contact with the aircraft, according to a Continental US North American Aerospace Defense Command Region news release.

The F-16s' speeds caused a sonic boom to reverberate across the Washington region, officials said, meaning they were traveling faster than the speed of sound, creating shock waves that caused a sudden and resounding boom.

Some residents on the ground reported being startled by the sound.

The US Capitol complex was placed on an "elevated alert" when the plane flew near the area Sunday afternoon, US Capitol Police said in a statement.

President Joe Biden, who was golfing near Maryland's Joint Base Andrews when the F-16s took off, was briefed on the incident, according to a White House official.