(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.
'Getting to the facts'
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.
Moore released a statement saying that "this is about getting to the facts and the truth, and we want the Johnson family and the community of Valdosta to have confidence in the process."
"I am cognizant of time, and we continue to move the process along," he said.
King and Crump, watched by a few dozen supporters of Johnson's family, made their comments on what would have been Johnson's 18th birthday.
Before the news conference, the supporters held signs calling for answers in the death, and chanted refrains like: "No justice, no peace!"
Johnson's father, Kenneth Johnson, spoke briefly, asking authorities to give his son "a good present today."
"Give us the videotape," Kenneth Johnson said.
Internal organs removed
The attorneys' plea also came after Johnson's parents told CNN this week that their son's body and skull had been stuffed with newspaper before burial.
That discovery was made during the second autopsy in June. The private pathologist, Dr. William R. Anderson with Forensic Dimensions in Florida, told CNN that every internal organ from the pelvis to the skull was gone when he opened up the teen's remains.
"We have been let down again," Kenneth Johnson told CNN. "When we buried Kendrick, we thought we were burying Kendrick, not half of Kendrick."
During an autopsy, internal organs are generally removed and examined before being returned for burial.
GBI spokeswoman Sherry Lang told CNN that after the first autopsy, "the organs were placed in Johnson's body, the body was closed, then the body was released to the funeral home."
The funeral home would not comment to CNN. But in a letter to the Johnsons' attorney, funeral home owner Antonio Harrington said his firm never received the teen's organs.
Harrington wrote that the organs "were destroyed through natural process" due to the position of Kendrick Johnson's body when he died, and "discarded by the prosector before the body was sent back to Valdosta." A prosector dissects the body for pathological examination.
King said Thursday that his team would take a close look at what happened to the body between the first autopsy and the burial.
CNN's Devon Sayers and Matt Smith contributed to this report.