A pilot and three passengers were killed Sunday after the unresponsive private jet they were on board crashed in a heavily wooded area near Waynesboro, Virginia, the Federal Aviation Administration said.

Crash investigators are looking at hypoxia -- a shortage of oxygen in the blood -- as a possible reason why the pilot and passengers didn't respond to attempts by air traffic controllers and other civilian aircraft to contact the plane, a source told CNN. Cerebral hypoxia can happen if there is a loss in cabin pressure or the plane reaches too high an altitude. The higher the altitude, the faster a person loses oxygen.

The FAA lost contact with the Cessna 560 Citation V, which flew from Elizabethton, Tennessee, past its destination -- New York's Long Island MacArthur Airport -- and turned back before eventually crashing in Virginia on Sunday afternoon, according to NORAD and FlightAware.

Here's what we know about the victims:

Adina and Aria Azarian

The private aircraft was registered to Encore Motors of Melbourne, Inc., a company based in Florida, according to FAA records. John Rumpel told CNN on Sunday night he and his wife, Barbara, own the company.

Rumpel told The Washington Post that Adina Azarian is his daughter and Aria Azarian is his 2-year-old granddaughter and they were among the three passengers and pilot that died in the crash.

Rumpel told the newspaper he received a call from the FAA about 90 minutes after dropping his daughter, granddaughter and their nanny at a Tennessee airport. They were headed home to East Hampton, New York, he said.

The FAA asked Rumpel if he knew how to contact the plane, the Post reported.

John Rumpel confirmed to CNN, his wife, Barbara Rumpel was safe but declined to comment further.

"My family is gone, my daughter and granddaughter," Barbara Rumpel wrote in response to a post on her Facebook profile in which others were asking if she was on the plane.

Lakhinder Jit Singh Vohra met Azarian a few years ago at a social event in the Hamptons and the pair became fast friends, he told CNN.

Vohra described Azarian as a private person who was very independent. He said she would always check on him and helped him when he launched a cigar brand.

"She was a genuine person that believed in collecting interesting people in her life and she lived every minute of it," he said. "A social butterfly, beautiful lady, a great mom ... very classy, no drama lady."

Azarian's employer, Keller Williams, remembered her in a statement as "a very devoted mother" and exceptional colleague.

"Her vibrant personality and unwavering commitment to her clients set her apart in the real estate industry. Adina's passion for her work and her genuine care for others touched the lives of many, making her an invaluable asset to our team at Keller Williams Points North," the company's New York City branch said in a statement.

Jeff Hefner

John Rumpel also identified the pilot as Jeff Hefner, according to the report in The Washington Post.

Hefner's previous employer, the head of a law firm where Hefner worked as a flight captain, said the pilot is survived by his wife and three children.

Hefner was "a highly accomplished and skilled Aviator, he flew 25 years with as a captain with Southwest Airlines and had over 25,000 flight hours," attorney Dan Newlin told CNN in a statement. "After retiring from Southwest Airlines, Jeff went on to be certified as a Captain in numerous private aircraft," he added.

Hefner was also remembered by the Southwest Airlines Pilots Association.

"Captain Jeff Hefner was a servant and an advocate throughout his life as a professional pilot. As a former committee and Board of Directors member at SWAPA, Jeff was a defender of his fellow pilots' safety, careers, and family. We offer our deepest condolences to his wife, his family, and his friends. The aviation community has lost a true champion. Tailwinds, Jeff," the association said in a statement.