Seventeen-year-old Kendrick Johnson was found dead and rolled up in a gym mat at Lowndes County High School in Georgia on Jan. 11, 2013.

State and local investigators initially ruled his death an accident, from "positional asphyxia." They concluded he’d climbed into the mat, gotten stuck and suffocated. And the Justice Department found insufficient evidence to prove a federal crime.

But eight years later, a new sheriff has reopened the investigation, and the doctor who conducted an independent autopsy told Fox News Wednesday that it was scientifically impossible for the teen to have died as initially described.

FILE- In this Dec. 13, 2013 file photo, Kenneth and Jacquelyn Johnson stand next to a banner on their SUV showing their late son, Kendrick Johnson in Valdosta, Ga. A Georgia sheriff said Wednesday, March 10, 2021, he's reopened an investigation into the 2013 death of the teenager, whose body was found inside a rolled-up gym mat at his high school. (AP Photo/Russ Bynum, File)

FILE- In this Dec. 13, 2013 file photo, Kenneth and Jacquelyn Johnson stand next to a banner on their SUV showing their late son, Kendrick Johnson in Valdosta, Ga. A Georgia sheriff said Wednesday, March 10, 2021, he's reopened an investigation into the 2013 death of the teenager, whose body was found inside a rolled-up gym mat at his high school. (AP Photo/Russ Bynum, File)

"Originally it was called ‘positional asphyxia,’" said Dr. William Anderson. "They felt he crawled in down the gym mat and into the middle of it, to try to get a sneaker -- which of course doesn’t make any sense because all you’d have to do is knock the gym mat over on its side, and the sneaker would be at the bottom."

Positional asphyxia, Anderson said, happens when someone is trapped in a way that they can’t breathe properly, then suffocates as their muscles grow exhausted and can’t overcome the pressure on their chest. It can take hours, and the evidence would be obvious, he said -- but the state's autopsy showed Johnson's lungs were "basically normal."

"To make that diagnosis, you need to have the evidence of the ongoing respiratory distress," he said. "Otherwise, the person doesn’t die."

GEORGIA SHERIFF REOPENS INVESTIGATION INTO DEATH OF KENDRICK JOHNSON

Anderson’s independent autopsy found the cause of death to be blunt force trauma to Johnson’s neck, likely caused during an altercation, he said.

"It could have been accidental in that they were putting pressure on his neck and didn’t mean to kill him," he said. "He certainly didn’t go into the gym and have positional asphyxia, because the hallmark findings for positional asphyxia aren’t there. So you can’t make a diagnosis, scientifically, if you don’t have the evidence."

Anderson said that state investigators were not interested in looking at his findings, which suggested homicide and not positional asphyxia.

A Georgia Bureau of Invesigation spokesperson said the bureau could not comment on the active investigation.

Current Lowndes County Sheriff Ashley Paulk, who is heading up the new investigation, has secured the case files from all of the state, local and federal agencies that investigated Johnson’s death in 2013. He said he planned to compare the evidence and check for inconsistencies.

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"We finally acquired, after many years of asking and begging and pleading, we got the federal files," he told Fox News Wednesday. "They were shipped to us on Feb. 11."

The DOJ said at the end of its investigation it had interviewed nearly 100 people, sorted through tens of thousands of emails or text messages and examined surveillance video from the school.

Federal authorities had previously declined Paulk’s requests for the documents, but the sheriff said that Johnson’s parents unwaveringly kept up the pressure.

"Really I give credit to the Johnson family," he said. "They persevered, and they found some allies in certain places, and they’re the ones that finally, in my opinion were, the determining factor."

Paulk, who previously served as sheriff for 16 years, had retired for medical reasons and left office before Johnson’s death.

But years after overcoming his heart problems, he decided to run for the job again, and he noted that "a lot of people have a lot of questions" about the eight-year-old case.

Now he’s looking to answer them.

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"And I’m gonna be involved of every inch of it," he said.

The sheriff said he had already conducted one interview personally in the new investigation and planned to at least be present for all of them.

Paulk said he expects the reopened investigation to take at least six months – and longer if the situation calls for it.