When Sybrina Fulton found out that her 17-year-old son, Trayvon Martin, had been shot dead by a self-appointed neighbourhood watchman in a gated community in Sanford, Florida, she made a decision: never to go there. She could not bear to see the place where Trayvon had been killed. Gripped by the “strange paralysis” of grief and unable to eat or sleep, Fulton lay in bed crying, waiting for her son’s body to be returned home. “Losing my child ripped my heart in half,” she writes, with typically unflinching honesty, in Rest in Power: the Enduring Life of Trayvon Martin. “It is indescribable pain. I could not get out of my bedroom.”

Days later, however, Fulton was packing a bag and heading from Miami-Dade, where she brought up her two sons and had worked for the county for 24 years, to Sanford. In that instant she went from grieving mother to civil rights activist. How did she summon the strength to do it? “I did it out of anger,” Fulton tells me. “When Tracy [Trayvon’s father, Tracy Martin] called me and said the police were not going to arrest the person who shot and killed Trayvon – although they had [him], they had the gun, and they were clear what had happened – I was in disbelief. That evening, I headed to Sanford. I could not believe they were going to let this person get away with murder.”

“It was shocking to know that someone can shoot and kill a 17-year-old unarmed child and the child goes to the morgue in a body bag and the adult goes home to his bed,” Martin adds. “It is also disturbing.”

Facebook Twitter Pinterest Trayvon Martin: ‘a very fun-loving, understanding young man with a beautiful smile’. Photograph: Mark St George/Rex Features

On the evening of 26 February 2012, Trayvon Martin was on his way home from a 7/11 convenience store with a can of iced tea and a bag of Skittles. It was raining, he had his hoodie on, and he was listening to music and talking on the phone as he returned to the house where he was staying with his father and his father’s girlfriend. (Fulton and Martin divorced when their children were young but continued to bring up their boys together.) He was followed, shot and killed by 28-year-old George Zimmerman. The details of the case, now infamous five years on, remain deeply shocking. The fact that Zimmerman followed Trayvon against the advice of the police. That he was recorded on tape saying, “Fucking punks. These assholes always get away”, yet claimed he acted in self-defence, invoking the stand-your-ground law that is currently enacted in 22 states. The fact that Trayvon’s body was immediately subjected to a drug and alcohol test, but Zimmerman was not. The fact that it took 44 days for an arrest to take place and then only as a result of a national media campaign. The fact that during the trial the prosecution was ordered to refrain from using the term “racial profiling”. And the fact that, in July 2013, Zimmerman was found (by a jury of six women, five of whom were white) not guilty of second-degree murder or manslaughter.

Rest in Power is a devastating and agonisingly detailed account of Trayvon’s life and death, and of the Black Lives Matter movement that followed. With Fulton and Martin taking consecutive chapters to tell the story, it unfolds like a true-crime serial: labyrinthine, episodic and appallingly real. Yet it is also the story of two ordinary parents, part of “the mass of Americans living an anonymous life of infinite complexities in the struggling suburb of an American city”, grieving for the loss of their child. They never intended to take five years to write it, but their anguish made it impossible until now. As Fulton quietly puts it: “We never imagined we would get there.”

“We were asked by our attorneys to keep notes from the beginning,” Martin explains. “And when we sat down and looked at everything that went on through the trial, we realised Trayvon wasn’t humanised through the court system. We don’t portray Trayvon as being an angel … but he was our angel. We don’t say we were the perfect parents, but we were his parents.”

“It was very difficult to write,” Fulton adds. “It was like reliving the story.”

Facebook Twitter Pinterest Demonstrators demand justice for Trayvon Martin, New York, July 2013. Photograph: Adrees Latif/Reuters

Since Trayvon’s death, and those of other black people including Michael Brown, Tamir Rice and Eric Garner, their collective legacy has been to reignite the race conversation in the US. Six days after the trial verdict, President Barack Obama delivered what many consider to be his most eloquent address about race. “When Trayvon Martin was first shot,” he began, “I said that this could have been my son. Another way of saying that is, Trayvon Martin could have been me 35 years ago.” Killings of young black men now make front-page news. The American Dialect Society declared #blacklivesmatter its word of the year in 2015. Racism, racial profiling (and gun control) were critical issues during the 2016 presidential campaign.

Yet since Fulton and Martin wrote the book, the US has changed again. How do they feel under the current administration? “That there is a lot of work to be done,” says Fulton, diplomatically. “I just think we have to do better here in the United States.” Along with other African American mothers who lost their sons to gun violence, she campaigned for Hillary Clinton during the 2016 presidential race. Is she more scared for black people now? “I can’t say I’m more scared,” she says, carefully. “We’re in a bad situation. I don’t know what Donald Trump’s plans are on gun violence. So I really can’t speak on that this minute.” Is Martin less hopeful now? “What I can say is there was hope with President Obama’s administration, and this administration has yet to prove that there is hope for the black man in America,” he says.

Meanwhile, the image of Trayvon in his hoodie continues to symbolise a sweeping and amorphous civil rights movement. The day before I speak to Fulton and Martin on the phone, on what would have been Trayvon’s 22nd birthday, his name, face and legacy are trending on social media again. “It reinforces the support,” says Fulton, “but at the same time it makes me feel sad, because he’s not here.” Fuelled by a deep and long-held sense of injustice, the Black Lives Matter movement pits itself against an institutionalised racism that means black people are imprisoned at nearly six times the rate of white people, and African Americans are more than twice as likely to be killed by police when unarmed than white Americans.

“As a parent and an African American man, I lost hope and faith in our justice system on the day the killer of our son was acquitted,” says Martin, who with Fulton set up the Trayvon Martin Foundation in 2012. “So our fight now is to make sure there are no more Trayvon Martins across this country. We feel strongly that the laws that pertain to African Americans are different to the laws applying to other individuals. That’s a conversation that this country is afraid to have. With all these other killings and police brutalities, I feel as though a systematic racism is plaguing this country and we have to take the blindfolds off. Racism is alive and well in America.”

For Fulton, the message the not-guilty verdict continues to send to the US and the rest of the world is stark. “That it’s OK to shoot and kill someone who is unarmed and not be held accountable is an awful message to send our teenagers,” she says. “They are afraid. They’re afraid to walk down the street in their own community. They’re afraid to listen to music. They’re afraid to wear hoodies. They’re afraid to go to the park and play. They’re afraid to be stopped by the police. They don’t even feel like they’re going to make it to 21 years old. We can’t have that message.”

Facebook Twitter Pinterest Martin and Fulton with a copy of their book, Rest in Power. Photograph: Johnny Louis/FilmMagic

Neither of them wanted a life of activism. “I never would have chosen this for myself,” Fulton says as she prepares to head to a book signing. “I’m just making the best of a sour situation, like the song on Beyoncé’s album about making lemonade out of lemons. We are just average people and this could have happened to anybody. Trayvon could have been anybody’s kid.”

“I look back on the last five years and all the things we’ve gone through,” says Martin, who before his son’s death was a truck driver who had never been on a protest. “I would never have imagined myself being an activist, being the voice of the voiceless. This was forced upon us. I just wanted to be a father.”

Part of the point of Rest in Power is to answer the question that opens the book: “Who was Trayvon Martin?” In death, Fulton writes, “Trayvon Martin became a martyr and a symbol of racial injustice, a name and a face on T-shirts, posters and protest signs”. Yet, when Trayvon was alive, he was “simply a boy”. A boy known as “Crazy-Legs” by his parents because he would never sit still and who called his mother Cupcake because he thought she was “so sweet”. A boy who was keen to pursue a career in aviation, would talk to anybody, was rarely out of his grey hoodie, and who had struggles like any other teenager. “We strongly feel in our hearts that Trayvon was victimised in the courtroom,” Martin says. “We strongly feel he was on trial for his own murder. So we wanted everyone who had a different perception of Trayvon to understand that this is who he is.

“He was a very fun-loving, understanding young man with a beautiful smile,” Martin says. “He was very respectable. He obeyed the laws of the land. He knew how to handle conflict, if something happened with an authority figure, because we spoke to him about that. He was a mama’s boy and a father’s boy. He was our child and we’ll never be able to touch him again. That’s who Trayvon Martin was.”