A U.S. Marine refueling tanker crashed into a soybean field in rural Mississippi on Monday, killing at least 12 people aboard and spreading debris for miles, officials said.

Leflore (le—FLOR’) County Sheriff Ricky Banks told The Associated Press that officials were still searching for bodies after nightfall, more than five hours after the KC—130 spiraled into the ground about 85 miles (135 kilometers) north of Jackson in Mississippi’s Delta region.

“We’re still searching the area,” Mr. Banks said. “It’s hard to find bodies in the dark.”

Mr. Banks earlier told the Greenwood Commonwealth that 16 people were believed to be on board but would not confirm that information to the AP. Mr. Banks said officials had found at least 12 bodies, but couldn’t rule out that more had been or would be found.

Marine Corps spokeswoman Capt. Sarah Burns said in a statement that a Marine KC—130 “experienced a mishap” on Monday evening but provided no details.

Andy Jones said 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.”

Mr. Jones said the plane hit the ground behind some trees, 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, Mr. Jones said.

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

Mr. Jones said a man borrowed his cellphone to report to authorities that there were bodies across U.S. Highway 82, more than a mile from the crash site.

Greenwood Fire Chief Marcus Banks, no relation to the sheriff, told the Greenwood Commonwealth that debris from the plane was scattered in a radius of about 5 miles (8 km).

Mr. Jones said firefighters tried to put out the fire at the main crash site but withdrew after an explosion forced them back. The fire produced towering plumes of black smoke visible for miles across the flat region and continued to burn after dusk, more than four hours after the crash.

Aerial pictures taken by WLBT—TV showed the skeleton of the plane burning strongly.

“It was one of the worst fires you can imagine,” Mr. Jones said. He said the fire was punctuated by the pops of small explosions.

Officials did not release information on what caused the crash or where the flight originated.