A Navy corpsman and 15 Marines were killed when a military plane crashed in rural Mississippi as it was headed from North Carolina to California, a Marine spokesman said.
Marine Corps Major Andrew Aranda said the flight of the KC-130T originated on Monday from Marine Corps Air Station Cherry Point, North Carolina.
The plane was taking people and equipment to Naval Air Field El Centro, California, when it crashed in a soybean field near Itta Bena, Mississippi, on Monday afternoon.
Mr Aranda said many of the Marines were from a unit based in New York with active duty and reserve members.
The Marine Corps said personal weapons and small-arms ammunition were aboard.
It was the deadliest Marine crash - in the US or abroad - since 2005.
A witness said some bodies were found more than a mile away.
The Marines gave no immediate details on the cause of the crash.
The FBI joined the investigation, but Mr Aranda told reporters no foul play was suspected.
The KC-130 is used to refuel aircraft in flight.
Andy Jones said he was working on his family's catfish farm just before 4pm when he heard a boom and looked up to see the plane spiralling downwards with one engine smoking.
"You looked up and you saw the plane twirling around," he said. "It was spinning down."
Mr Jones said that by the time he and others reached the crash site, fires were burning too intensely to approach the wreckage.
The force of the crash nearly flattened the plane, he 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 mobile phone to report to authorities that there were bodies across a highway, more than a mile from the crash site.
Greenwood Fire Chief Marcus Banks told the Greenwood Commonwealth that debris was scattered in a radius of about five miles.
Mr Jones said firefighters tried to put out the fire but withdrew after an explosion forced them back.
The fierce blaze produced black smoke visible for miles across the flat region and continued to burn after dusk, more than four hours later.