(CNN) — The private plane that crashed in Virginia on Sunday, after US fighter jets tried unsuccessfully to make contact with it, left a “crater” in the ground, no survivors and few clues as to why it went down, officials said Monday.

The four people on board – one pilot and three passengers – were all killed in the crash, according to a Federal Aviation Administration report.

Four first responders, who spoke on the condition they were not identified, described a grisly scene at the crash site.

There were perhaps four recognizable pieces of wreckage from the plane, which they believe impacted the ground at a very steep angle, they said. “There was nothing really bigger than your arm,” said one.

They also found signs of human remains.

Federal officials are investigating the crash of the unresponsive plane that flew near the US Capitol region on Sunday, prompting military fighter jets to rush to intercept the aircraft before it went down in Virginia, leaving no survivors, authorities say.

National Transportation Safety Board officials will spend the next few days processing evidence at the crash site, before the wreckage is helicoptered to a secure facility in Delaware, lead investigator Adam Gerhardt told reporters Monday.

The plane was not required to carry “black boxes,” devices used to record flight data, but investigators will still search for them in hopes that they were installed.

Gerhardt said at this very early stage in the investigation, “basically everything is on the table,” when it comes to determining the cause.

The civilian aircraft flew from Elizabethton, Tennessee, past its destination – New York’s Long Island MacArthur Airport – and then turned back before eventually crashing in Virginia on Sunday afternoon, according to NORAD and LiveATC.net.

Air traffic controllers, fighter jets, and even other civilian pilots frantically tried to contact the unresponsive crew of the Cessna jet by radio as it barreled toward Washington, DC, at 34,000 feet, audio from LiveATC.net revealed.

No surviving passengers were found at the crash site Sunday night, Virginia State Police said, noting their identities will be released when they become available.

The details of why the plane veered so far off course and what caused the crash are still unclear. First responders said the crash site is amid steep, mountainous terrain that is difficult to reach on foot.

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

F-16 fighter jets were “authorized to travel at supersonic speeds” as they raced to make contact with the aircraft, a Cessna 560 Citation V, according to a news release from the Continental US North American Aerospace Defense Command Region.

The jets’ extraordinary speed caused a sonic boom across the Washington, DC, area, officials said. That means the jets were traveling faster than the speed of sound, creating shock waves that caused a sudden and resounding boom, startling some residents on the ground.

“The house rocked, the windows rattled, and it was done, like that. We were obviously freaked out a little bit,” Chuck Martin, who felt the boom in Fairfax County, Virginia, told CNN affiliate WJLA. “I started calling the police. Their nonemergency line was busy forever. When I finally got through, they said, ‘Oh, this DOD aircraft went supersonic, which sounded a little bit strange.’”

Residents in Maryland also reported hearing the boom.

“Our Ring notifications were going crazy with so many different options. Was it the Navy? Or was it a sonic boom? Or was it an earthquake?” Sandy Abuarja, who lives in Gambrills, Maryland, told CNN affiliate WBAL.

Others were concerned about where the sound came from, particularly because of their proximity to the nation’s capital. “It was really quiet, and then we woke up, and it was like a really sudden … It didn’t shake the ground, but it was a little too loud,” Gambrills resident Ahreum Kindess told WBAL. “I was like, ‘Oh my gosh, I live right next to DC. What was that?’”

When the F-16s reached the Cessna around 3:20 p.m., the jet pilots set off flares in an effort to get the pilot’s attention, the release said.

“The pilot was unresponsive and the Cessna subsequently crashed near the George Washington National Forest, Virginia,” the release said. “NORAD attempted to establish contact with the pilot until the aircraft crashed.”

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

State and local police in Virginia searched the area for hours after being notified of a possible crash near the northern city of Staunton, the agency said.

No survivors found in the wreckage

None of the plane’s passengers were found alive when first responders reached the crash site around 8 p.m. Sunday night, state police spokesperson Corinne Geller confirmed.

The private aircraft is registered to Encore Motors of Melbourne, Inc., a company based in Florida, according to FAA records.

John Rumpel, whose wife Barbara is listed as the president of the company, told CNN they own Encore. The husband confirmed Barbara Rumpel is safe, but declined to comment further.

John Rumpel’s daughter, a 2-year-old granddaughter and her nanny were onboard the plane, Rumpel told The New York Times. The family was returning home to East Hampton, New York, after a four-day trip to his home in North Carolina, he told the newspaper.

“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.

Plane flew over US capital area

As the aircraft veered off course Sunday afternoon, it passed over Washington, DC, and was not responding to efforts to make contact, prompting the F-16 fighter jets to respond.

US Capitol Police placed the Capitol complex on an “elevated alert” as the aircraft passed through the region, the agency said.

“This afternoon, our officials were working closely with our 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,” a statement from the department said.

It’s unclear whether the aircraft entered restricted airspace.

President Joe Biden was golfing at the Andrews Air Force Base golf course near Maryland’s Joint Base Andrews when the sonic boom resounded through Washington. The US Secret Service said it did not alter its posture for keeping President Biden secure after the fighter jets were scrambled.

The president was briefed on the incident, according to a White House official.

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