A slim, pale man in T-shirt and trainers walks up to a police officer, his head slightly bowed, and appears to ask a question. After a couple of seconds he makes a fumbled grab for the officer’s gun, then quickly gets pushed to the floor. The crowd around him start to holler and cheer.

So begins the unlikely and upsetting story of The Brit Who Tried to Kill Trump – a new BBC documentary about Michael Sandford, the young man from Surrey with a number of mental health problems who attempted to shoot Donald Trump at a Las Vegas rally in 2016.

The 18-year-old Michael had moved to America the year before to be with a girlfriend he had met online. Despite the attempts of his mother, Lynne, to stop him and then to stay in touch, Michael gradually lost contact with his family until, in June last year, Lynne received a phone call from the Foreign Office to inform her that her son had been arrested. The documentary, which was filmed in one month and released on BBC iPlayer the day after it was finished, to coincide with the presidential inauguration, follows the Sandford family as Michael stands trial in the US.

A few minutes into the film, we see Lynne in a woolly jumper and thick glasses talking down the phone from Dorking to her son about what he has eaten that day. “Just potatoes,” we hear. No porridge today. It seems mundane until you realise Michael is speaking from a detention centre in Nevada, where he is being held for the attempted assassination of the man who is now US president.

“Although it was a fairly high-profile story at the time, it was three or four months later that I realised this kid went to my school,” explains documentary-maker Guy Simmonds. “Suddenly, I had a personal connection. I found out where Lynne lived, travelled there and knocked on the door. She wasn’t in, so I scribbled a note on an envelope, explaining that I had gone to the same school as Michael and that it would be great to talk. We struck up a relationship from there.”

Speaking to Lynne, and watching her on film, I am struck by her quiet, matter-of-fact courage. She talks with an almost dispassionate honesty about Michael’s mental health. “He does suffer with seizures and he has autism,” she says. “Over the years, he’s had a lot of visual and auditory hallucinations that he kept secret; he was paranoid someone would perform brain surgery on him if we knew. He lived in fear for years. The psychiatrist who did an in-depth study of Michael in the States basically said he had worn himself out to the point where he couldn’t resist any more.”

‘He’s finding it incredibly difficult to cope. He’s had death threats’ … Lynne Sandford, Michael Sandford’s mother. Photograph: BBC

We hear in the documentary about Michael’s near-fatal anorexia, of his being sectioned, about Lynne’s struggle to get him proper preventive treatment, of his vulnerability and his being bullied. “With autism, people are often very logical, factual, everything is black and white. But OCD [which Michael also has] is almost the opposite – it’s completely illogical,” she says. “So he’d feel compelled to do all these rituals, but at the same time he’d say: ‘I know how ridiculous this is.’”

Lynne paints a picture of a man suspended between his strong, rational sense of the world, and his own irrational, emotional response to stressors. How is that playing out in prison? “He’s finding it incredibly difficult to cope,” she says. “He’s in the protective custody unit for his own welfare. He’s had death threats. There are a lot of inmates and guards – Trump supporters – who are threatening to do something to him. He’s absolutely petrified.”

Because of his mental illness, Michael wasn’t placed in the general prison population and is spending 20 hours a day in his cell. He has been under suicide watch on and off since he went into the detention centre, though not since his sentence was passed [he received 12 months]. “On suicide watch, you’re stripped of clothes,” says Lynne. “You’re not even allowed toilet paper in case you try to choke yourself.”

Michael may be moved to another prison soon, to serve out the final months of his sentence. Such a change could prove both disruptive and dangerous.

It would be natural for any parent, facing this sort of situation, to look for a fault in themselves, a provocation, a cause. But in Michael’s case, no such explanation seems available. “He was so different to the boys we knew,” says his father, Paul, in Portsmouth. “Neither Lynne or I are hugely political. I’ve only ever voted a couple of times, because my view is that they’ll all be ungrateful buggers, whoever gets in. Michael never even watched the news. So you can’t help but wonder where these thoughts came from. His condition is probably something to do with it. But I can’t help but think that he met someone who basically groomed him. Though we’ve got no proof.”

Judge James Mahan, who tried Michael’s case, was rather more specific, if surprisingly sympathetic, at his sentencing: “You have a medical problem. You should not be ashamed or embarrassed about it. You need medication. You’re not a hardened criminal,” he said. “You’re not evil or a sociopath like a lot of people we have. I don’t think you wanted to kill anybody. This was just some crazy stunt that your mind told you to do.”

In the footage Simmonds gathered for the documentary, we see Michael, sitting on the ground outside the rally, being questioned by police. In the clip, he seems quietly, but adamantly, fixed on the problem: that Trump is a racist. It’s a sentiment many people may share. But why did this young man go further than most would dare imagine. Was this a suicide attempt? Did the violent pitch of American political discourse throw him into such distress that he lost control? Did the widespread access of firearms play a part? Was he being used? Had he been radicalised? Was it a cry for help? Until Michael is released from prison, and gets the long-term support Lynne has always campaigned for, we may never know.

Was he being used? Had he been radicalised? Was it a cry for help? … Michael Sandford. Photograph: BBC

“The thing that struck me as being missing from the film was the phenomenal effort Lynne put into trying to get Michael help,” says Paul. “In the last few years she was regularly up until 2am emailing people, researching, making appointments. Although we’re not together any more, my concern is that people might criticise her or ask what she was doing to stop this happening. Nobody could have done more.”

Simmonds hopes the film will create discussion about funding for mental health services. “As NHS resources become increasingly stretched by government cuts, we do see mental health services struggle to cope,” he says. “There’s also this difficult crossover period around 18 years old when someone passes from youth to adult services and people can get lost in the system.”

When I ask Lynne why she agreed to be in the documentary, she tells me it’s because Michael wanted her to. “He said he would like the chance to tell his story,” says Lynne. “Otherwise the world will just see him as this bad person.” She pauses. “I think Michael was just a very lost, troubled, misguided soul and he’d come to the end of being able to cope,” she says at last. “The only person Michael has ever hurt is himself.”

The Brit Who Tried to Kill Trump is available now on BBC3 on the iPlayer and is on BBC1 on 29 January at 11.20pm.