Sheriff Paulk said he examined 17 boxes of evidence presented by prosecutors and investigators, including grand jury testimonies. The evidence “provides nothing to prove any criminal activity by anyone that would have resulted in the death of Kendrick Johnson,” Sheriff Paulk’s report said.

Kendrick’s father, Kenneth Johnson, said he still believes someone killed his son, a popular athlete at Lowndes High and the youngest of five siblings.

Mr. Johnson, 53, said a family forensic pathologist, William R. Anderson, determined the cause of death was blunt trauma to the right side of the boy’s neck near the jaw.

“The truth is the truth,” said Mr. Johnson. “You can’t turn a murder into an accident.”

The results of Mr. Anderson’s autopsy, conducted five months after Kendrick’s death, contradicted the conclusions of the Georgia Bureau of Investigation coroner, who issued an autopsy report in 2013 that found the cause of death to be “positional asphyxia,” suggesting it Kendrick was pinned headfirst in the rolled mat and suffocated.

Sheriff Paulk said the conflicting autopsy reports were complicated by a third autopsy performed by the Office of the Armed Forces Medical Examiner in 2014. This report also ruled that the death was accidental and caused by asphyxia, but was later amended to say the cause of death was “undetermined.”

Even after the federal investigation, questions about Kendrick’s death lingered in Valdosta.

In 2016, Sheriff Paulk, who had retired a year before Kendrick’s death, decided to run again for his old seat. He said he was approached several times during the campaign by black voters who wanted him to revisit the case.

“I felt like I should watch that,” he said. “Also, I wanted to get to know myself.”

But Sheriff Paulk, 76, said when he reviewed federal documents, it appeared prosecutors were determined to indict the two students. He described their investigation as a “witch hunt” and said email correspondence between prosecutors shows they felt pressure to get results.