WASHINGTON (AP) — The crash of an F-35B Joint Strike Fighter aircraft in South Carolina over the weekend has raised numerous questions about what prompted the pilot to eject and how the $100 million warplane was able to keep flying pilotless for 60 miles (100 kilometers) before crashing.

Here’s what is known about the modern warplane and its latest incident:

‘FORCED TO EJECT’

A U.S. Marine Corps pilot was flying a single-seat F-35B fighter jet on Sunday when the pilot experienced a malfunction and was “forced to eject,” a Marine Corps official who was not authorized to speak publicly said on condition of anonymity. The aircraft was only at an altitude of about 1,000 feet (300 meters) and only about a mile (less than 2 kilometers) north of Charleston International Airport, in a populated area that led the pilot to parachute into a residential backyard.

The Marine Corps’ variant of the F-35 is different from the Air Force and Navy versions in that it can take off and land like a helicopter — which allows it to operate on amphibious assault ships. But it’s also different in that it’s the only one of the three variants that has an auto-eject function on its ejection seat, according to seat manufacturer Martin-Baker. That has raised questions as to whether the malfunction the pilot experienced was the seat itself.

On the Air Force and Navy versions, “the pilot has to initiate the ejection,” said Dan Grazier, a former Marine Corps captain and the senior defense policy fellow at the Project on Government Oversight, but the Marine version’s auto-eject is intended to better protect the pilot in case something goes wrong with the aircraft when it’s in hover mode. “Was that function triggered for some reason, and punched the pilot out?” Grazier said. “There’s a lot of unanswered questions.”

Last December, an F-35B that had not yet been delivered to the Marine Corps crashed at Naval Air Station Joint Reserve Base Fort Worth in Texas. The jet had been in hover mode over the airfield and began to drop, hit the runway and bounced before the pilot was ejected into the air.

In July 2022, the Air Force temporarily grounded its F-35s over ejection seat concerns. While the Air Force F-35A does not have an auto-eject function, some of the cartridges that initiate the ejection in the warplane were found to have issues, leading to the grounding.

At the time, all F-35 ejection seats, including the Navy and Marine Corps variants, were inspected, and the continue to be looked at during standard maintenance on the aircraft, the F-35 Joint Program Office said in a statement to The Associated Press.

THE AIRCRAFT KEPT FLYING

Other major questions include how the aircraft continued flying for 60 miles before crashing in a field near Indiantown, South Carolina, and why the pilot bailed out — if the bailout was intentional — of a plane that was able to keep operating for that long, said Mark Cancian, a retired Marine Corps Reserves colonel and senior adviser at the Center for Strategic and International Security.

“If it flew that far, could (the pilot) have landed it someplace — why punch out where he did?” Cancian asked.

The search for the aircraft lasted more than a day before the debris was ultimately located Monday by a South Carolina law enforcement helicopter.

A Marine Corps official said he could not provide any additional details on why it took so long to find the jet, citing the ongoing investigation. Jeremy Huggins, a spokesperson at Joint Base Charleston, told NBC News that the jet was flying in autopilot mode when the pilot ejected from the aircraft. Once it was located, a Marine Corps team was dispatched to secure the wreckage and a second team, one that conducts aircraft mishap investigations, was sent to the site.

FIGHTER JET OF THE FUTURE

The Lockheed Martin-produced F-35 Lightning Joint Strike Fighter is the most advanced fighter jet in the U.S. arsenal, with more than 972 warplanes already built and plans to produce more than 3,500 globally. The Defense Department is counting on it serving for decades as the primary fighter both for the U.S. and a host of allied partners, much like the role the F-16 Flying Falcon was designed to fill decades ago.

The jet “represents so much of the future” of the country’s airpower, Cancian said.

It was almost 22 years ago that Lockheed won the contract to build the F-35. It created three variants — the Air Force’s F-35A, which is the most produced version and the one most often sold to allies; the Marine Corps F-35B, which has the ability to take off and land vertically, and hover like a helicopter; and the Navy’s F-35C, which can land on a carrier.

Lockheed Martin has delivered 190 F-35B variants to the Marine Corps, at a cost of about $100 million each.

The program, however, has faced significant cost overruns and production delays, and its final price tag now tops an estimated $1.7 trillion. While many of the aircraft have been built, the program is already looking at replacing the F-35 engine.