An 18-year-old student, Nolan Wells, was found dead off the Mississippi coast on 6 July after a boat trip with friends. Preliminary findings from an independent autopsy, released on Wednesday, have left the cause of his death undetermined.

The examination, conducted by forensic pathologist Dr Roger Mitchell at the request of the family, found no fractures or deep tissue injuries, but noted a “red discoloration” in the soft tissue at the back of the head, with no corresponding skin lacerations or skull fractures. The report stated that advanced decomposition of the body made it impossible to determine whether bruising had been present, and that parts of the throat had been retained by the initial official autopsy, preventing a full assessment of possible neck injury. The independent review did not include toxicology testing; the official toxicology report from the Mississippi State Medical Examiner’s Office has not yet been made public.

The findings have been interpreted differently. At a news conference in Chicago, the family’s attorney, Ben Crump, said the pathologist “cannot rule out if any intervening non-accidental factors contributed to or caused the death.” By contrast, a former New York City chief medical examiner, Dr Michael Baden, told US media that the head discoloration was consistent with a minor, possibly post-mortem impact from debris in the water and did not point to foul play. In Jackson County, Mississippi, sheriff’s officials have said initial interviews suggest Wells chose to stay on the island and that his friends believed he would find another ride; they have stated they do not suspect a crime, though the investigation remains open.

The case has drawn national attention from civil rights leaders, who have questioned why the teenager’s phone and keys were taken by the friends and why he was not reported missing immediately. The Rev Al Sharpton, who delivered the eulogy at Wells’s funeral in Ocean Springs, Mississippi, has cautioned against ruling out foul play before the investigation is complete, citing the state’s history. A reward of $125,000 for information leading to an arrest and conviction has been offered. The FBI is assisting local authorities, and the district attorney has said the case will be presented to a grand jury. The official autopsy results are still awaited.