U.S. fighter pilots who intercepted a Cessna plane that flew over restricted airspace in Washington, D.C., on Sunday found the pilot "slumped over" before the business jet crashed in a remote mountainous area in Virginia, killing all four people on board, Fox News has learned.

National Guard F-16 pilots confirmed the civilian pilot was unresponsive after trying to get the pilot’s attention for 30 minutes before the plane went down around 3 p.m. near the sparsely populated town of Montebello, Virginia, military officials said.

"The Federal Aviation Administration confirmed that the pilot did not respond to air traffic control instruction around 1:28 p.m. EDT," North American Aerospace Defense Command (NORAD) said in a statement. "Subsequently, the NORAD pilots visually inspected the Cessna as it was still airborne and confirmed that the pilot was unresponsive. NORAD pilots described the Cessna pilot as being slumped over."

Air traffic control audio from the half-hour before the Cessna Citation crashed captured voices that identified themselves as military pilots trying to communicate with the pilot of the private plane, The Associated Press reported, citing recordings on LiveATC.net.

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Rescue teams at Cessna plane crash

Search and rescue teams leave the command post at St. Mary's Wilderness en route to the Blue Ridge Parkway to search for the site where a Cessna Citation crashed over mountainous terrain near Montebello, Virginia, on Sunday. (Randall K. Wolf via AP)

"If you hear this transmission, contact us," said one pilot who identified herself as being with the Air National Guard. Several minutes go by without a response when a military pilot said, "You have been intercepted. Contact me."

To intercept the Cessna aircraft, the U.S. deployed six F-16 jets, including two from Washington, D.C., two out of New Jersey and an additional two out of South Carolina, officials said. A supersonic boom heard across the Washington, D.C., metropolitan area was from one of the D.C. National Guard F-16 jets trying to catch up with the Cessna.

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While authorities have not confirmed the identities of those aboard the plane or what caused the crash, the plane’s owner, John Rumpel, told The New York Times and Newsday that his daughter, Adina Azarian, and 2-year-old granddaughter Aria, were aboard the flight.

Barbara Rumpel, her daughter and granddaughter

Barbara Rumpel, pictured left, wrote on Facebook that her daughter, rightmost, and granddaughter, center, were "gone" following an outpouring of condolences after a Cessna plane crashed in Virginia with four people on board. (Barbara Rumpel/Facebook)

Azarian, 49, was well-known in real estate circles both in New York City and Long Island, described by friends and relatives as a competitive entrepreneur who started her own brokerage and was raising her daughter as a single parent.

Photo of Adina Azarian

Adina Azarian and her 2-year-old daughter Aria were two of the four people killed in a plane that crashed in a remote part of Virginia on Sunday, according to Azarian's father, John Rumpel. The pilot and a reported nanny were also killed. (Lakhinder Vohra via AP)

Rumpel, a pilot who runs the Florida-based company Encore Motors of Melbourne Inc., to which the crashed plane was reportedly registered, said his family was returning to their home in East Hampton, on Long Island, after visiting his house in North Carolina.

National Transportation Safety Board investigator Adam Gerhardt said the wreckage is "highly fragmented." He said the plane is not required to have a flight recorder but that it may have other avionics equipment with data that they can examine.

Sheriffs in Augusta County, Virginia

Authorities secure the entrance to Mine Bank Trail, an access point to the rescue operation along the Blue Ridge Parkway where a Cessna Citation crashed over mountainous terrain near Montebello, Virginia, on Sunday. (Randall K. Wolf via AP)

Investigators are now seeking answers as to how long the pilot could have been unresponsive and what caused the wayward aircraft to turn around over Long Island and head back toward Tennessee.

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Experts theorize that the plane could have lost pressurization, causing hypoxia, a condition that occurs when someone’s brain is deprived of adequate oxygen.

Fox News' Liz Friden and The Associated Press contributed to this report.