The FBI is investigating the Monday crash of a U.S. Marine Corps plane in Mississippi that killed at least 16 people and is believed to have had a “structural failure” at 20,000 feet. The KC-130 refueling tanker had departed from Memphis before crashing near a highway in rural Leflore County at around 4 p.m. The Marine Corps confirmed only that a “mishap occurred” and has yet to release further details, including its destination and if any civilians were on board. Witness Andy Jones told a local Fox News affiliate that he saw the plane corkscrew out of the sky with one engine burning, before it crashed in soybean fields. “You looked up and you saw the plane twirling around,” he said. “It was spinning down.” Firefighters struggled to carry out rescue and recovery efforts due to several “high intensity explosions” near the wreckage, Greenwood Fire Chief Marcus Banks told The Commonwealth newspaper. The explosions were thought to be caused by ammunition on the plane. Unidentified investigators were also cited in local media saying the scale of the debris suggests a possible mid-air explosion, though that theory has not been confirmed.