Military investigators picked through the charred wreckage of a Marine Corps cargo plane Tuesday, seeking clues on why it spiraled out of the sky in western Mississippi, killing 15 Marines and one Navy sailor, said a Marine statement.

The dead included six Marine raiders and a sailor that were traveling to conduct pre-deployment training at the Marine Corps air station in Yuma, Ariz, according to a Marine Corps Special Operations Command statement released Tuesday afternoon. All seven service members belonged to 2nd Marine Raider Battalion.

“The incredible demands of this dangerous and demanding calling forge some of the tightest unit and family bonds found in the U.S. military,” the statement said. “This loss impacts us all.”

The small, tight-knit Marine Special Operations community has been hit especially hard by aviation accidents in recent years. In 2015, seven of the elite troops, also from 2nd Marine Raider Battalion, died when they were flying aboard an Army UH-60 Blackhawk helicopter that crashed during a training exercise off the Florida panhandle.

The probe into Monday’s crash of the KC-130 aircraft, en route from Marine Corps Air Station Cherry Point, N.C., was ongoing, the Marine statement added. The plane is used for refueling and carrying cargo and troops. Equipment onboard the aircraft at the time of the crash included small arms and ammunition, another Marine statement said. Bomb disposal experts have been dispatched to the scene as a precaution.

The crash appears to be the worst aviation accident for the Marine Corps since a CH-53 transport helicopter went down in Iraq in 2005, killing 30 Marines and one sailor.

The names of those killed were not immediately made public pending notification of family members.

Mississippi Emergency Management Agency Director Lee Smithson told the Clarion-Ledger that the aircraft crashed on the Leflore-Sunflower county line, a rural stretch of woods and fields with dense underbrush and vegetation about 85 miles north of Jackson. Earlier, Sheriff Ricky Banks told the Ledger that five of the crew members were confirmed dead.

Andy Jones, a local resident, told the Associated Press that he was working on his family’s catfish farm just before 4 p.m. when he heard a boom and looked up to see the plane corkscrewing downward with one engine smoking.

“You looked up and you saw the plane twirling around,” he said. “It was spinning down.”

He told the AP that the plane hit the ground behind some trees in a soybean field, and by the time he and other reached the crash site, fires were burning too intensely to approach the wreckage. The force of the crash nearly flattened the plane, Jones said.

“Beans are about waist-high, and there wasn’t much sticking out above the beans,” he said.

Pictures posted to Facebook and published by the Ledger showed thick black smoke coming from a field. The debris field appears to span a large area.

In a statement Tuesday morning the commandant of the Marine Corps, Gen. Robert Neller, pledged a thorough investigation of the incident.

“On behalf of the entire Marine Corps, I want to express my deepest condolences to the families of those killed in the aircraft mishap yesterday afternoon in Mississippi,” Neller said.

The KC-130 is a four-engine propeller-driven aircraft that is a variant of the Cold War-era C-130 Hercules. The plane can be configured to refuel aircraft in midair or equipment on the ground. It can also haul troops and equipment and in some cases carry an array of weapons.

Sen. John McCain (R-Ariz.) paid tribute to the 16 marines who died after a U.S. military transport plane crashed in Mississippi on July 11. (Reuters)

Brian Murphy and Dan Lamothe contributed to this report.