ITTA BENA, Miss. — Investigators picked through debris across a fire-blackened soybean field Tuesday to try to determine why a U.S. military plane slammed into the ground, killing all 16 people aboard in the deadliest Marine crash anywhere in the world in more than a decade.

The KC-130 air tanker was carrying members of an elite Marine special operations unit cross-country for training in Arizona when it went down Monday afternoon in the Mississippi Delta, the military said. The fiery crash scattered wreckage for miles around and sent a pillar of black smoke rising over the countryside.

Witnesses said they heard low, rumbling explosions when the plane was still high in the sky, saw the aircraft spiraling toward the flat, green landscape and spotted an apparently empty parachute floating toward the earth.