JACKSON, Miss. — The military transport plane that slammed into soybean fields Monday in the Mississippi Delta, killing 15 Marines and a Navy sailor, appears to have developed problems while high in the air, a Marine general said Wednesday.

“Indications are something went wrong at cruise altitude,” Brig. Gen. Bradley S. James told reporters Wednesday in Itta Bena, Mississippi. That squares with comments from witnesses interviewed by The Associated Press who said they saw the plane descend from high altitude with an engine smoking.

The crash of the KC-130 killed nine Marines from Newburgh, New York, and six Marines and a Navy Corpsman from Camp Lejeune in North Carolina, James said.

James said that there is a “large debris pattern,” including two main impact areas separated by a mile, with a four-lane highway in between them.

Mississippi Public Safety Commissioner Marshall Fisher repeated earlier warnings that people in the crash area shouldn’t pick up any debris, which could include weapons, ammunition and evidence valuable to determining why the plane crashed.

“None of that stuff should be touched,” Fisher said. “Removal of anything from the area could be subject to criminal prosecution.”

Fisher, who also spoke at the news conference, said the federal Bureau of Alcohol, Tobacco, Firearms and Explosives, as well as federal prosecutors in northern Mississippi, are investigating reports that someone removed debris. State law enforcement agencies are guarding the area, but the broad area and number of roads makes it difficult to control access.

Fisher urged people to call the ATF at 1-800-ATF-GUNS if they find anything.

Leflore County Sheriff Ricky Banks said the debris is spread across 2 to 3 miles (3 to 5 kilometers) of farmland. He estimated Wednesday it will take investigators five or six days to sift through the wreckage and clean up the site where the plane crashed on Monday.

Six of the Marines and the sailor were from an elite Marine Raider battalion at Camp Lejeune, North Carolina, and were headed for pre-deployment training in Yuma, Arizona, the Marine Corps said Tuesday.

Marine Maj. Andrew Aranda said Wednesday the names of those killed will not be released until 24 hours after family members are notified.

Several bouquets were left Tuesday at the main gate of Stewart Air National Guard Base in Newburgh, New York, where the plane was based. Stewart was closed to reporters and did not issue a statement.

“We’re feeling the pain that everybody else is,” Robert Brush said after dropping off three pots of red, white and blue petunias. He works for a landscaping company that serves the base.

It was the deadliest Marine Corps air disaster since 2005, when a transport helicopter went down during a sandstorm in Iraq, killing 30 Marines and a sailor.

The crash happened outside the small town of Itta Bena about 85 miles (135 kilometers) north of the state capital of Jackson.

The Marine Corps said the cause was under investigation and offered no information on whether the plane issued a distress call.

FBI agents joined military investigators, though Aranda told reporters no foul play was suspected.

“They are looking at the debris and will be collecting information off of that to figure out what happened,” Aranda said.

The KC-130 is used to refuel aircraft in flight and transport cargo and troops.

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 spiraling downward with one engine smoking.

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

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, Jones said.

Catfish farmer Will Nobile said he drove to the site and as he and others stood by a highway, they saw an open parachute wafting down from the sky: “It didn’t look like anybody was in it.” Another catfish farmer found an empty, open parachute later near a fishpond, Nobile said.

The Marines said the plane was carrying personal weapons and small-arms ammunition — equipment that may have contributed to the explosion and the popping that could be heard as the wreckage burned.