Johnson, 17, was found dead in a rolled-up gym mat at Lowndes High School in Valdosta, Ga., earlier this year, CNN reported Friday.
U.S. Attorney for the Middle District of Georgia Michael Moore issued a subpoena for the school's original hard drives containing hundred of hours of surveillance footage.
Chevene King, the family's attorney, has questioned whether footage previously supplied by the school had been altered, alleging time codes weren't shown.
"We don't have any time code with which to synchronize the events that are shown in the video. ... Either the cameras did this on their own or a human being interacted to make these cameras do these things," King said.
However, Lowndes County Schools denied the allegations, saying the footage was "a raw feed with no edits."
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