There's not enough evidence to support federal criminal charges in the case of a teenager found dead inside a rolled-up gym mat at a Georgia high school, the U.S. Department of Justice announced on Monday.

The decision comes after a lengthy review of circumstances surrounding the death of 17-year-old Kendrick Johnson of Valdosta.

Local and state authorities had ruled the teenager's death Jan. 10, 2013, was a freak accident. They concluded that Johnson got stuck upside down in the middle of a rolled up mat and was unable to breathe.

Johnson's parents insisted someone must have killed their son and have pushed to reopen the investigation.

"After extensive investigation into this tragic event, federal investigators determined that there is insufficient evidence to prove beyond a reasonable doubt that someone or some group of people willfully violated Kendrick Johnson's civil rights or committed any other prosecutable federal crime," the Justice Department said in a statement Monday. "Accordingly, the investigation into this incident has been closed without the filing of federal criminal charges.

Classmates at Lowndes High School found his body on Jan. 11, 2013, inside one of the rolled-up gym mats propped upright against a wall next to the gymnasium bleachers.

The strange circumstances of Johnson's death attracted attention far beyond his hometown of Valdosta near the Georgia-Florida state line. Rallies and marches calling for answers drew up to 200 people, while thousands followed social media sites dedicated to the case. The Florida lawyer who represented Trayvon Martin's family worked to help ensure Johnson's death wasn't forgotten.

Sheriff's investigators for Lowndes County closed the case four months after Johnson turned up dead. They concluded he died in a freak accident while reaching for a gym shoe inside one of the mats. A state medical examiner ruled the cause of death was "positional asphyxia," meaning the teenager got stuck upside down in a position that left him unable to breathe.

Johnson's parents and their supporters refused to let the case be put to rest. They got a judge's order to exhume the body last summer so a second autopsy could be performed. The private pathologist hired by Johnson's family reported finding hemorrhaging beneath the skin of Kendrick's jaw and neck and concluded he suffered a fatal blow near his carotid artery that appeared to be "non-accidental."

Sheriff's detectives who investigated the death insisted they found nothing to suggest foul play. But the 522-page case file revealed enough stumbles and loose ends to fuel conspiracy theories.

Though Lowndes High School has dozens of surveillance camera monitoring its doors and hallways, including four cameras in the gym where Johnson was found dead, none of them recorded the events leading to his death. Investigators suspect the gym mats were too far away to trigger any of the motion-activated cameras, which filmed Johnson strolling into the gym but nothing else. Attorneys for Johnson's parents suggested footage released to the family and the media may have been altered.

The case file notes that the county coroner wasn't called to the scene until six hours after Johnson's body was found. Under Georgia law he should have been notified immediately. The coroner criticized sheriff's investigators for the delay, but agreed with their conclusion that Johnson's death was accidental.