At least an hour of footage is missing from each of the four surveillance cameras at Kendrick Johnson’s high school, and the original footage showing how the Georgia teenager died might be gone, CNN reported on Thursday.

“Those files are not original files,” forensics video analyst Grant Fredericks told CNN. “They’re not something investigators should rely on for the truth of the video.”

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CNN enlisted Fredericks in order to analyze more than 290 hours of video it acquired from Lowndes County High School, where the 17-year-old was found dead in the gym in January. Local officials initially determined that he died from asphyxiation after getting trapped inside a gym mat, an argument his family has rejected. An independent autopsy ordered by the victim’s family found the cause of death to be “unexplained, apparent non-accidental, blunt force trauma.”

But while the Lowndes Public Schools district told CNN the video it provided was “a raw feed with no edits,” Fredericks disagreed, saying it was “altered in a number of ways, primarily in image quality and likely in dropped information, information loss. There are also a number of files that are corrupted because they’ve not been processed correctly and they’re not playable. I can’t say why they were done that way, but they were not done correctly, and they were not done thoroughly. So we’re missing information.”

Specifically, two of the cameras are missing 65 minutes of footage each, while the other two are missing 130 minutes apiece. Another camera outside of the gymnasium has a time stamp 10 minutes behind the ones inside.

Fredericks also criticized the Lowndes County Sheriff’s Office for requesting a copy of the gym footage from school officials. CNN reported that the request was not fulfilled until five days after Johnson was found dead.

“Right now, what they’ve done, is they’ve left it up to the school district as to what it is they want to provide to the police, and I think that probably is a mistake,” Fredericks told CNN. “You don’t want somebody who might be party to the responsibility to make the decision as to what they provide the police.”

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Johnson’s death is also being investigated by U.S. Attorney Michael Moore, who announced on Oct. 31 that “a sufficient basis exists for my office to conduct a formal review of the facts.”

Watch CNN’s report on Fredericks’ review of the footage, aired on Thursday, below.