(NewsNation) — Limitations encountered in the second autopsy of Mississippi teen Nolan Wells — including missing body parts and inexplicable skin discoloration — are not considered unusual when a forensic pathologist reexamines a corpse after the first postmortem, professionals say.

Dr. Roger Mitchell, who was retained by Wells’ family to do an independent autopsy, said he could not determine a cause of death because portions of the teen’s neck and internal organs had been retained from the first autopsy. As a result, he said he could not rule out “nonaccidental factors” in the death.

The 18-year-old went missing after traveling July 4 to an island off the coast of Mississippi with a group of friends who returned without him. Wells’ body was recovered two days later from the water. His family has demanded answers about what happened, and an investigation continues.

In his report, Mitchell said the missing organs and neck structures made it impossible for him to determine if Wells had suffered a neck injury or whether water had entered his lungs. He also noted “red discoloration” in the back of Wells’ skull that he could not explain.

Forensic pathology experts say second autopsies are fraught with limitations that include removal of evidence and significant changes to the body.

Retaining neck structures in an autopsy is routine “for further radiographic examination,” the National Association of Medical Examiners says in a position paper on second autopsies. Even if internal organs remain, they typically have been replaced inside the body with no relation to their original biological order.

“Some dissected organs and tissues will remain in the body cavity (commonly in a biohazard bag), but these materials will be extensively comingled,” NAME says.

Other challenges of a second autopsy include “progression of decomposition” and discolorations that can be misidentified.

“Routine manipulation of the body during a first autopsy may result in extravasation of blood into soft tissue that can be misinterpreted as bruising or hemorrhage during a second autopsy,” the NAME document says.

On the plus side, the professional organization says a second autopsy is effectively a “peer review” that can help reinforce findings or correct errors in the first procedure. NAME recommends cooperation between the public medical examiner and independent reviewer, “if legally permitted.”

The Mississippi State Medical Examiner has not released its autopsy results in the Wells case. Toxicology tests can often take weeks.

The Associated Press contributed to this report.