Kendrick Johnson's parents believe his death was no accident

(CBS News) A federal prosecutor in Georgia said Thursday he will conduct a formal review into the death of a teenager whose body was found in his high school gym. His parents believe his death was no accident.

It was Jan. 10 when surveillance cameras at a Valdosta, Ga., high school captured video of Kendrick Johnson walking a hallway and then later strolling alone across the gymnasium floor.

It was an unremarkable moment.

But sometime in the next 24 hours, the 17-year-old three-sport athlete died.

His body was found Jan. 11 inside a rolled-up wrestling mat that was propped on its end behind some bleachers in the gym.

A Georgia coroner ruled the death accidental and police theorized Johnson suffocated after falling headfirst into the rolled-up mat trying to retrieve a sneaker.

But his family suspected foul play. They had his body exhumed and hired a private pathologist, who reported finding evidence of "blunt force trauma" on his neck.

Now Kenneth and Jacquelyn Johnson want justice for their son.

"I really believe he was murdered. You can tell by the way he looked with bruises on his face," Kenneth Johnson said.

The Johnson family has hired Benjamin Crump, the same attorney who worked with the family of Trayvon Martin, the African-American teenager who was shot and killed in Florida in a confrontation with Neighborhood Watch volunteer George Zimmerman.

Supporters of Johnson's family have held rallies alleging a cover-up. They've demanded a federal investigation.

On Thursday, U.S. Attorney Michael Moore agreed to look into the case.

"I do this with an open mind, neither accepting nor rejecting the opinions of anyone who has previously investigated the circumstances of his death," Moore said.

Moore says if he finds evidence of a possible crime, he'll press the FBI to investigate. But the Georgia chief medical examiner is standing by his finding that the death was accidental, saying there is no evidence the teenager was murdered.