4 dead after plane that prompted F-16 response in D.C. crashes

• Adam Gerhardt, National Transportation Safety Board (NTSB) senior air safety investigator, said it's "a very challenging accident site" and the wreckage is highly fragmented.
• Investigators will look at "the airplane, the engines, the weather conditions, pilot qualifications, the maintenance records — all aspects will be of course items that we routinely look at," Gerhardt added.

What we know: A sonic boom rang out across the D.C. region as fighter aircraft responded to an unresponsive pilot flying a Cessna.

• NORAD said it deployed an F-16 fighter jet to intercept the plane, which the Federal Aviation Administration reported was carrying four people before it later crashed in mountainous terrain near Montebello, Va.
• State and local law enforcement reached the crash site by foot just before 8pm after an hourslong search in the Staunton-Blue Ridge Parkway area, per an emailed statement from Virginia State Police spokesperson Corinne Geller late Sunday.
• "No survivors were located," she said.
• The FAA and the NTSB are both investigating the incident that saw startled residents across the region post on social media about hearing a "loud explosion" as the sonic boom occurred.

Zoom in: Officials in Maryland said the boom was caused by an authorized Defense Department plane out of Joint Base Andrews, where President Biden was playing golf with his brother, according to a pool report.

• A White House official said Biden was briefed on the situation, per a pool report. "The sound resulting from the authorized DOD aircraft was faint at JBA," the official added.

What they're saying: The North American Aerospace Defense Command said in an emailed statement Sunday evening that the civilian aircraft was intercepted at about 3:20pm ET.

• "The NORAD aircraft were authorized to travel at supersonic speeds and a sonic boom may have been heard by residents of the region," per the NORAD statement.
• "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.
• "The pilot was unresponsive and the Cessna subsequently crashed near the George Washington National Forest, Virginia," NORAD added. "NORAD attempted to establish contact with the pilot until the aircraft crashed."

Meanwhile, the U.S. Capitol Police said in an emailed statement Sunday evening that its officials worked closely with federal partners to "monitor an unresponsive pilot" who was flying an airplane near the National Capital Region.

• "The U.S. Capitol Complex was briefly placed on an elevated alert until the airplane left the area," the statement added.

What's next: The FAA and NTSB are working to determine what caused the Long Island, New York-bound Cessna Citation 56o to crash after it left Elizabethton, Tenn.

• The NTSB will release a preliminary report on the crash within 10 days, Gerhardt said during a Monday press conference. He said a final report detailing the probable cause and more can take anywhere between a year to two years to be released.

Between the lines: A sonic boom is "an impulsive noise similar to thunder," according to the U.S. Air Force.

• "It is caused by an object moving faster than sound — about 750 miles per hour at sea level," per the Air Force.
• Sonic booms can shatter glass, but the threat to communities is low.

Editor's note: This article has been updated with new details throughout.