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.