A teenager who was beaten by guards in a state-run boot camp for juvenile delinquents died from internal bleeding caused by a blood disorder, not from injuries he may have suffered in the beating, a medical examiner reported Thursday.

Martin Lee Anderson suffered from sickle cell trait, which caused his red blood cells to change shape and produce "a whole cascade of events that led to bleeding and hemorrhaging," said Bay County Medical Examiner Dr. Charles Siebert.

"It was a natural death," he said.

Medical forms filed at the boot camp did not indicate that the youth or his family were aware of the disorder, which affects about one of every eight black people, Siebert said. The sickle cell disorder would not show up in routine blood tests, he said.

Anderson entered the camp Jan. 5 because of an arrest for theft, and complained of breathing difficulties and collapsed during exercises that were part of the entry process. He died the next day at a Pensacola hospital.

The Bay County sheriff's office, which runs the camp, said Anderson became uncooperative and was restrained. The boy's mother contends he was murdered by guards who beat him to death.

Siebert said there were some bruises and abrasions on the boy's body, but he attributed those to attempts to resuscitate the youth.

Also Thursday, the Justice Department announced it would investigate the case, along with the FBI. Federal officials planned to focus on whether camp guards violated Anderson's rights through use of excessive force or "indifference to serious medical need," U.S. Attorney Gregory R. Miller said.

On Friday, law enforcement officials planned to make public a videotape that allegedly shows guards beating Anderson. News organizations have filed a lawsuit seeking the video.