America’s recent focus on 22-year-old Gabby Petito, whose body was found last month in a remote part of Wyoming, revived criticism of “missing white woman syndrome” and the media’s failure to convey that Black lives matter too.

There are few starker example than Kendrick Johnson, an African American teenager found dead inside a rolled-up gym mat at his high school gymnasium in 2013. How and why he lost his life remains a mystery that has never been solved.

“Kendrick went missing: nobody searched for him,” his mother, Jacqueline Johnson, says to the Guardian by phone from Valdosta, Georgia. “Nobody called a US marshal or the FBI for him. Nobody called all those people in. They didn’t put it all over the news. They didn’t do none of it.

“You see the imbalance right there. If a Black person goes missing it’s just like they don’t really care. As soon she [Gabby Petito] goes missing, because of her colour, she gets all the media, all the shows, everything. Everywhere you see Ms Petito. Nobody else matters.”

Local and state authorities ruled 17-year-old Kendrick’s death a freak accident, concluding that he got stuck in the middle of the mat while reaching for a gym shoe and was unable to breathe. Jacqueline and her husband Kenneth insist that he was murdered, accuse law enforcement and school officials of a cover-up and continue to wage a campaign for the truth.

The case is the subject of a new documentary, Finding Kendrick Johnson, based on a four-year undercover investigation into a tangled web of allegations, new evidence and alarming questions over authorities’ handling of the tragedy. With didactic narration by actor and activist Jenifer Lewis, the film demands that attention must be paid.

Kendrick was a sports-loving teenager. Jacqueline, 49, recalls: “He was the baby of our family. He was just like the funniest person ever. He played three sports all the way up through high school: he played football and basketball and he ran track. Football was his passion in life. Older kids looked up to him because he would say, ‘Yeah, one day I’m going to make it in football. Y’all going to see.’”

The bright shard of memory darkens. “They just took his dream away and they took our goal away because that was our goal for him. He loved football. I would say two or three years old, he was out there playing football with his dad and brothers, and then when he was old enough to get on the field, he was playing football ever since he was four years old.”

When she thinks of Kendrick now, Jacqueline says, “it’s so much hurt, so much pain. We can’t even really begin to grieve or mourn because of the fight we’ve had to endure.”

Fellow students found Kendrick’s body in the gymnasium at Lowndes high school in Valdosta – a town in the deep south where Confederate monuments still stand - at around 10.30am on 11 January 2013. Kendrick was 5ft 10in; the rolled up mat is 6ft tall. The mat was propped upright against a wall next to the gym bleachers.

Sheriff’s investigators closed the case four months later, concluding that Kendrick died in a bizarre accident while reaching for a shoe inside one of the mats. A state medical examiner ruled the cause of death was “positional asphyxia”, meaning the teenager got stuck upside down in a position that left him unable to breathe.

But Kendrick’s family were left with many unanswered questions. There were strange gaps in surveillance footage from the gym. The county coroner was not called to the scene until six hours after Johnson’s body was found; under Georgia law he should have been notified immediately.

The family secured a judge’s order to exhume the body so that a second autopsy could be performed. A private pathologist found haemorrhaging beneath the skin of Johnson’s jaw and neck and concluded he suffered a fatal blow near his carotid artery that appeared to be “non-accidental”.

The second autopsy made another shocking discovery: Kendrick’s internal organs were missing and paper had been stuffed into the cavities (the fate of the organs remains unknown).

In October 2013 a federal criminal civil rights investigation was announced, leading to interviews with almost a hundred people, but the justice department closed the case in June 2016, saying investigators “found insufficient evidence to support federal criminal charges”.

The Johnson family also pursued various civil lawsuits without success; they were even countersued and face potential legal costs of hundreds of thousands of dollars.

“My whole family is hurt because they treated us like we’re the ones who have committed a crime and instead we’re the victims,” Jacquelyn reflects. “We’re Kendrick’s mother and father and family and friends, and they treated us like we’ve committed a crime. It’s hurtful and it’s so wrong on so many levels.”

Brian and Branden Bell, white classmates of Kendrick, were reportedly considered persons of interest in the FBI investigation. The brothers were never charged and an FBI video analysis determined they were in different parts of the school when Kendrick went to the gym. (Their father, Rick Bell, was an FBI agent who resigned after his house was raided for evidence.)

The brothers told investigators and the media that they did not see Kendrick on the day of his death. But the documentary dug up surveillance footage showing Kendrick standing about four feet from Brian Bell in a school hallway, as well as an unredacted FBI report that states: “Kendrick Johnson is observed in a covered walkway moving towards the camera. Bell is observed in the same location at this time.”

The Bell brothers deny any wrongdoing.

The film’s writer director, Jason Pollock, first met Jacquelyn when he was living in Ferguson, Missouri, during the uprising that followed the death of Michael Brown, a Black teenager fatally shot in 2014 by a white police officer (the subject of his film Stranger Fruit). After becoming Facebook friends with Jacquelyn, Pollock noticed graphic pictures of Kendrick’s mutilated face cropping up daily in his feed.

As the film highlights, Jacquelyn’s willingness to share the harrowing image of her son evokes Emmett Till, a 14-year-old Black boy lynched in Mississippi in 1955, whose mother decided to have an open casket so the world could see what had been done to him. The killers were never brought to justice but the searing image was galvanising for the civil rights movement.

Jacquelyn believes that someone first posted the image with malicious intent. “I would never have shared no photo of him looking the way he looked – as a mother, it hurts me that you would see that picture – but I began to share that and remind the people this is what they did to my child.”

Moved and incensed, Pollock went on to spend four years researching Kendrick’s death, determined to show that it is a burning injustice long overdue a fresh look. “If a white kid had been rolled up in the gym that day, I wouldn’t have had to do this,” he says by phone from California.

“We wouldn’t even be talking right now because the system would have done its job and the system didn’t do its job from day one. They wanted to make this look like it was an accident because they didn’t want to do an investigation.”

Pollock continues: “The whole story is just Shakespearean, what they’re going through, watching the state abuse them like this. The film doesn’t come to a conclusion. We’re not trying to support any conspiracy theories but we’re just trying to show the facts that we’re giving the light of day.

“I hope this piece of art puts a human face on the trauma as well as just gets the story out there. We don’t know what happened. But this is a story we should be talking about because I do believe he is our generation’s Emmett Till.”

The director, who started his career working for the documentarian Michael Moore, has become a fierce advocate for the family. “We are calling on President Biden and his team to review the malpractice at the FBI and the DoJ with the Kendrick Johnson case and unless Washington DC starts looking at this case, the state of Georgia is never going to deal with this.

“If you want to clean up the act of our nation, you need to clean up this case because this is a really, really, really bad one and this family deserves way more from America than they’ve gotten. If you care about Black Lives Matter issues, you need to take the Kendrick Johnson case on in Washington and deal with this.”

Finding Kendrick Johnson opens with whistleblower Mitch Credle, a 23-year veteran homicide detective with Washington police who investigated Johnson’s death with the US attorney’s office. He says by phone: “I hope the film will wake up law enforcement as far as wanting to look at it again. What bothers me at this point is: who will look into it?

“No one’s going to really trust the local jurisdiction there to look into it because of, I guess, the bad feelings, the history. The federal government were recused from the situation. So I don’t know who they will get to come in and look at it and just go based on the facts.

“Don’t do it with any villains involved. Just go and investigate it based on the evidence, regardless of who the suspects are. Just start from A and investigate it the proper way. All the families ask for is just do things the proper way. That’s all they want.”

Earlier this year Ashley Paulk, a county sheriff in Georgia, reopened the investigation into Kendrick’s death, saying he was reviewing 17 boxes of evidence collected by federal agents that his office obtained from the US justice department.

Jacqueline is sceptical of that effort but says: “I remain hopeful. I’m going to stay hopeful and I’m going to stay prayerful that one day the truth will come out.”

The memory of Kendrick is with her constantly. But so is the manner of his death. If “closure”, whatever that means, is even possible, it still eludes her. “There’s no days of not thinking about him and the things that happened to him and how can people be so evil? This is eight years’ worth of pain and it is’s growing stronger and stronger every day. They say it gets easier, but it don’t.”

• Finding Kendrick Johnson is available to rent now in the US with a UK date to be announced