Story highlights Source: U.S. attorney issued grand jury subpoena for school hard drives

Kendrick Johnson's body was found in a rolled-up gym mat in January

Previously released video was missing footage from high school gym

Forensic video analyst: Hard drives may have been overwritten by now

The FBI was expected Thursday to seize the original hard drives from the surveillance system at Lowndes High School, where 17-year-old Kendrick Johnson was found dead in a rolled-up gym mat earlier this year, according to a source with knowledge of the investigation surrounding the hard drives.

U.S. Attorney for the Middle District of Georgia Michael Moore issued a grand jury subpoena to seize the hard drives, the source said.

Kendrick Johnson's family waited months for hundreds of hours of surveillance video, hoping it would yield answers about their son's January death, but they were disappointed to learn that the four cameras inside the Valdosta, Georgia, gymnasium showed only a few collective seconds of Johnson, jogging. The camera fixed on the gym mats was blurry.

The Johnsons' attorneys were not shy in stating their suspicion that someone could have tampered with the videos.

Attorney Chevene King has questioned why time codes weren't shown in the videos.

"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.

Lowndes County Schools insisted last month that the video it provided to the family and CNN is "a raw feed with no edits," and the county sheriff's office, which asserts Johnson accidentally died while reaching for a shoe in one of the mats, says it didn't edit any files, according to their lawyers.

CNN, which filed suit to secure access to the video, hired forensic video analyst Grant Fredericks to analyze more than 290 hours of material from all 35 cameras inside and outside of the gym. Fredericks is a U.S. Justice Department consultant and contract instructor for the FBI National Academy in Quantico, Virginia.

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CNN also provided Fredericks and his company, the Spokane, Washington-based Forensic Video Solutions, with hundreds more hours of video from 31 cameras in other parts of Lowndes High School.

Fredericks quickly knocked down most of the Johnsons' concerns -- they're all easily explained, he said -- but his examination raised what could be another mystery: at least an hour of missing video from all four cameras inside the gym.

Fredericks told CNN he found that "highly suspicious," especially considering how the material was acquired by police.

CNN has requested access to the original surveillance servers, which has not yet been granted. But Fredericks cautioned that the video could be gone, as newer surveillance would replace it if it wasn't recovered promptly from the school's digital video recorder.

The files given to him by CNN are not original, Fredericks has said. "They're not something investigators should rely on for the truth of the video."

The analyst added: "The investigator's responsibility is to acquire the entire digital video recording system and have their staff define what they want to obtain."

Authorities have said they didn't receive a copy of the videos until several days after Johnson's body was found, according to an unredacted report obtained by CNN after a legal process.

According to an incident report from the sheriff's office, a detective watched a portion of the video, then asked an information technology officer employed by the school board to produce a "copy of the surveillance video for the entire wing of the school with the old gym for the last 48 hours."

Five days later, the sheriff's report says, the IT officer delivered a hard drive to the detective, who verified it contained what he requested.

"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 said.

"You don't want somebody who might be party to the responsibility to make the decision as to what they provide the police."

Jacquelyn Johnson, Kendrick Johnson's mother, said of the case, "We're going to fight until it's all over, until we get the truth."