Many questions remain after an unresponsive plane crashed in Virginia after flying over restricted airspace in Washington, D.C. on Sunday. The plane, a Cessna 560 Citation V aircraft, was a private jet registered to a Florida-based company owned by John and Barbara Rumpel. John Rumpel has said his daughter, 2-year-old granddaughter and a nanny were on board. 

All three passengers and the pilot, who remained unresponsive during the ordeal, died and the plane crashed near George Washington National Forest in Virginia. The cause of the incident is being investigated by the Federal Aviation Administration and National Transportation Safety Board. 

What caused the crash?

It is unclear why the pilot was unresponsive, but CBS News aviation safety analyst Robert Sumwalt, former chairman of the National Transportation Safety Board, said the plane likely continued to fly on autopilot after the pilot became incapacitated due to hypoxia. 

"There have been events where the airplane has depressurized or not pressurized and so everybody on the airplane gets hypoxic," he said. At higher altitudes, the air is thinner and has less oxygen. People aboard an airplane can get hypoxia if the plane doesn't correctly pressurize, first falling asleep and eventually dying. 

This happened in 1999, when golfer Payne Stewart's chartered jet lost pressure and could not obtain oxygen. The plane flew on autopilot for four hours and crashed near Aberdeen, South Dakota. All six people on board died. 

The appearance and severity of hypoxia is caused by several factors, including the rate of ascent and even a person's physiological fitness, according to the FAA. It can't be predicted, but it can be prevented by flying "a well-maintained pressurized airplane" as well as flying at an altitude where oxygen is not required. A pilot's inability to detect hypoxia is blamed for many incidents, according to the FAA. 

Hypoxia has also occurred on commercial flights before. Sumwalt cited a 2005 crash where a 737's crew did not set the pressurization correctly and the airplane never pressurized, so it flew until it ran out of fuel and crashed in Greece. All 115 passengers and six crew members died, according to the FAA.

If a pilot is incapacitated, a jet could fly on autopilot until it runs out of fuel, which Sumwalt said likely happened in the recent incident in Virginia. 

Sumwalt, a retired 737 captain, says many small planes don't require two pilots and the standard is based on the weight of the plane. 

"This jet itself requires two pilots, but with a waiver, a pilot can be a single-pilot operation," he said. If the airplane is equipped so that a single pilot can fly it alone and the pilot demonstrates they can fly the plane without a second pilot, a waiver may be granted, Sumwalt said. 

It is unclear if the pilot had a waiver to fly alone, but it is possible for this model to be flown alone, with FAA certification. 

How high and fast can a private jet fly?

Commercial jets usually reach 35,000 feet but small private jets can fly up to 41,000 feet or even higher, according to multiple private plane brokers.

The Cessna Citation V, which, like its smaller counterpart the Cessna Citation II, seats eight passengers, can reach 45,000 feet. 

How expensive are private jets?

On one private jet rental site, the price to rent a Cessna Citation V is $3,250 per hour. The price to buy one could be as steep as $2.2 million – plus the price of maintenance, fuel and the crew. In 2016, Aircraft Owners and Pilots Association said the average price for a 1989 model was $900,000 and the average for a 1994 model was $1.3 million.

Larger jets like a 747 cost hundreds of millions of dollars. The average cost to rent a Boeing 747 is $30,950 per hour.

How common are private plane crashes?

Last year, the Aircraft Owners and Pilots Association, which advocates for aviation, recorded 968 accidents involving non-commercial planes. Nearly 160 of those accidents were fatal and 264 people died. 

Just 21, or 2.2%, of the accidents were determined to be pilot-related, but a whopping 826, or 85.3%, have yet-to-be-determined causes.

About 200 of the recorded accidents of non-commercial aircraft involved private planes, such as the one involved in this incident. And of all the recorded accidents, only 70 had a second pilot on board. 

After the unresponsive plane was flown over restricted airspace, the U.S military deployed fighter jets to intercept the plane, which caused a sonic boom in the area. The military monitored the plane until it left the area and a U.S. official told CBS News that the Cessna was not shot down by the F-16s.

While there are far fewer aviation fatalities compared with automobile fatalities, private aircraft have a greater chance of crashing than commercial airliners, according to FAA data. Charter pilots are under much less regulatory scrutiny than commercial pilots and commercial planes have more protections in place, including frequent maintenance, checks and training for pilots and crew. 

Pilot error accounts for more than half of fatal airplane crashes. Because of this, the FAA limits the flight time of commercial crews and requires rest periods. 

Caitlin O'Kane is a digital content producer covering trending stories for CBS News and its good news brand, The Uplift.