(CNN) -- Attorneys for the family of a 17-year-old student found dead in a rolled-up gym mat at a southern Georgia high school called on authorities Thursday to release surveillance video that they say could show what happened.
Kendrick Johnson, they allege, was slain in January -- counter to authorities' conclusions that the three-sport athlete suffocated in the mat while reaching for a sneaker.
"There is one eyewitness that we know is available -- it is the video recordings made from surveillance cameras there in the gymnasium where the body was found," attorney Chevene B. King Jr. told reporters in Valdosta, Georgia.
"For some unknown reason, this tape has been withheld," he added.
Attorney Benjamin Crump, who recently joined the case after representing the family of slain Florida teenager Trayvon Martin, said that Johnson was "murdered, and we intend to get to the truth of what happened."
Johnson was found dead at Lowndes County High School in Valdosta on January 11, his body resting headfirst in the rolled wrestling mat.
A Georgia Bureau of Investigation autopsy found that he died from positional asphyxia, and the Lowndes County Sheriff's Office determined the death was accidental.
But a second autopsy, which a private pathologist conducted at the request of Johnson's parents in June, found that he died because of "unexplained, apparent non-accidental, blunt force trauma."
The Lowndes County sheriff has declared the case closed, and the U.S. Justice Department said in September that it wouldn't open a civil rights investigation. But Michael Moore, the U.S. attorney for the district that includes Valdosta, is reviewing the case and weighing whether to open his own investigation.
King acknowledged Thursday that Moore's office has surveillance material from the school and that he believes Moore is "making every effort to establish what in fact the ... tapes may show."
Still, King said, he doesn't know whether Moore has all the relevant tapes, and he said his team should be permitted to see them -- which he said hasn't yet happened.
"I'm (awaiting) some word from (Moore) as to whether or not" he'll release the tapes to the team, King said.
The attorneys, flanked by a few dozen supporters of Johnson's family, made their plea on what would have been Johnson's 18th birthday.
Before the news conference, the supporter held signs calling for answers in the death, and chanted refrains like: "No justice, no peace!"
"We will solve this murder mystery. We will get to the truth," Crump said.
CNN's Victor Blackwell, Devon Sayers, Matt Smith and Jason Hanna contributed to this report.
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