How did Samantha Lewthwaite end up on the run from Interpol after being linked to a series of terror attacks? As attention turns to Britons fighting with Isis in Syria, whatever happened to the young woman known as ‘the white widow’?

What radicalises a person from the UK? What makes anyone leave the certainties of Cardiff for the privations of Syria? How could any extremist preacher guess that Buckinghamshire would be fertile ground, not just for disaffected youth, but for disaffected youth with an undiscovered sense of purpose?

As terrorism experts pore over the videos released by Isis, and the education secretary fights with the home secretary about whose job it was to “drain the swamp”, the film-maker Adam Wishart has undertaken a detailed analysis not of context but of character: that of Samantha Lewthwaite, known as “the white widow”, once married to the 7/7 bomber Germaine Lindsay, now on the run from an arrest warrant in Kenya and an Interpol red notice, for suspected possession of explosives and alleged membership of al-Shabaab.

What he discovers is a person simultaneously bizarre and mundane, affected yet artless. Wishart says: “I’m always struck by how banal it all is. All that narrative of terrorism is about hate preachers who brainwashed X. I think it’s much simpler than that. She is in this extreme place now. If you follow every step she took, they almost all make sense. If I think about my own life, if I had altered the trajectory of each decision I’d made over 20 years, then I too would be in a very different place. The one step she made that doesn’t make sense was when she began to believe that violence is the right course.”

It would be impossible to try to situate Lewthwaite within a movement, however ardently she would wish it. She wears her radicalisation so proudly, but it sits strange and ersatz upon her, like she bought it in Claire’s Accessories. When Lindsay killed 26 people and himself, on 7 July 2005, Lewthwaite was seven months’ pregnant with their second child; their first, a son, was 14 months old. For a while, she condemned his attack, notably in an interview with the Sun, but by 2008, in her search for a new husband, she was expressly looking for a jihadist – and found one. His identity is disputed but he’s known to be a Kenyan, father of her third and fourth children. He is since thought to have died while on a terrorist mission, and latest reports suggest Lewthwaite may have made a third match. “Allah,” she wrote, in notes seized by Kenyan police in 2011, “blessed me with the best husband for me. In fact, exactly what I asked for when I made du’a before marriage. I asked for a man that would go forth, give all he could for Allah and live a life of terrorising the disbelievers as they have us.”

Pages from notebooks found by police in Samantha Lewthwaite’s home in Mombasa

She had her third child in 2009 in Stoke Mandeville hospital in Aylesbury. Two years later – after having had a fourth child in Johannesburg – she turned up in Mombasa but had to leave in a hurry after she was tracked down by police. When officers arrived at her house they found a fake passport, a copy of Heat (the film) and about 60 pages that they took to calling her diary, even though part of it she clearly intends as a book: “I have for many years now wanted to write something that would benefit my brothers and sisters.”

Once, her conversion was the fascinating thing about her. Even though friends from her childhood and early adulthood won’t talk about her, the story emerges rather straightforwardly: a patchwork of teenage disappointment in her parents’ divorce, an intense friendship with a Muslim family across the road, maybe some enthusiastic religious lessons. Wishart says: “It’s not a simple brainwashing. She met this girl, she liked her family, she converted. The Muslim community in Aylesbury wouldn’t say that radicalisation came from here, but the radical preachers of London knew that people were travelling weekly from Aylesbury.

“Somebody probably introduced her to Germaine Lindsay, and they probably, I don’t know, pushed each other harder until he blew up a train. It’s a trajectory that is on the one hand explainable, even banal. You meet someone, you get influenced by their ideas, and it takes you further. Yet on the other, at the point he decided to blow up a train, and kill commuters, it is completely unfathomable.”

From this point on, especially after she left the UK, she became more than a curiosity; she was mythologised. The arrest warrant, the florid accounts of her possible involvement in terrorist attacks from a 2012 grenade attack in Mombasa to the Westgate massacre in Nairobi the next year – she is frequently described, albeit mostly by British tabloids, as at the top of the command structure.

Experts in jihadi terrorism are cautious about this idea. Dr Nelly Lahoud, of the Combating Terrorism Center at the US military academy, describes in fine detail the tensions of Lewthwaite’s likely place in al-Shabaab, or any similar organisation. “I have a question mark over whether she is the operative that everybody is making her out to be.” So-called “defensive Jihad”, which is the theological code under which all these stateless organisations fight, explicitly frees everyone from normal authority – child from parent, wife from husband, slave from master, debtor from creditor – in order to fight.

Under this law, women could and should fight alongside men. And yet, since jihadists find support among people who are socially conservative, this would be unthinkable, not least because it would involve a woman travelling or fighting with a man who wasn’t mahram (a male relative). Lahoud goes on: “So, for example, Ayman al-Zawahiri (leader of al-Qaida) always describes how he took the news of the death of his family, when the US invaded Afghanistan: when the compound was bombed, people rushed to help and a woman’s hand came out. A man approached, and she put her hand back in. That was his wife. He responded that ‘you lived with modesty and you died with modesty’. This was a source of pride, that she preferred to die rather than be saved by someone who was not mahram.”

This doesn’t mean necessarily that “the white widow” would have no place in the command structure: “Although ideologues are very clear, they do not wish to have women fighting on the battlefield. They still believe that women have a critical role to promote the cause of jihad.” And that role is not just as helpmeet, but also propagandist, morale raiser, the mother raising her children with the love of jihad. This crops up constantly in Lewthwaite’s notes. “Recently, my husband gave a talk to my eight-year-old son and my five-year-old daughter. What do you want to be when you are older? Both had many answers, but both agreed to one of wanting to be a Mujahid. He asked them, how did they plan to achieve such a goal, and what really is a Mujahid?… It was my husband’s talk to the kids that made it clear it was time to put pen to paper.”

Her writings slip between an affected, homily style, the stilted syntax of someone pretending their mother tongue is a second language, and the casual platitudes of a Facebook update. It often feels like a quest for nothing more than a believable domesticity: “When a man comes home – wife beautiful, food prepared, kids clean – immediately he will forget the pain of his day, he will always want to come home. When the wife would hear her hubby, she would take his coat and shoes, feed him.”

‘I asked for a man who would give all he could for Allah’ … Lewthwaite with her husband Germaine Lindsay who killed 26 people and himself on 7 July 2005 in the London bombings. Photograph: Rex

She starts to make notes on the “Search for Salvation”, but it peters out and runs into shopping lists and web addresses for furniture, a to-do list that ends, “make drinks, look fabulous, sexcee”. Lahoud notes that “there’s a lot of emphasis on support and welfare – she wants her kids to give gifts to the poor”. Lewthwaite concocts a parable about her family travelling to an orphanage during Ramadan, where her daughter gives out toys and makes friends, while she thrills to the gratitude. It sounds phoney and sad, as if all she wanted was a marriage and a life from the olden days, and it was more realistic to find it in a terrorist cell than to try to make it happen in Aylesbury.

“It’s clearly related to her identity,” Wishart says of her conversion, even (especially?) at its earliest point. “I made a film about animal rights extremists once, and one thing that was clear was that once they’d got into this world, they couldn’t get out. Obviously, they have just crossed the road. But the ideas of animal rights had become so entwined with their identity that they couldn’t disentangle. There’s more than that here – Samantha got lots of acclaim, interest and excitement that she would probably never have got otherwise.”

Alexander Meleagrou-Hitchens, head of research at the International Centre for the Study of Radicalisation, explains how inexorable that trajectory can be for the convert, from Aylesbury to east Africa. The real leap is to radicalise. “Once they’ve bought into this idea that their primary duty is to fight in this world, al-Shabaab can say: there are fronts in the West, you can launch an attack there, but the more important front is where we are fighting to establish Sharia law. Your number one duty is to make sure manmade law is no longer running the show.”

After the 7/7 bombings, within violent circles Lewthwaite’s status would have increased, while her place in mainstream society would have been for ever tainted by Lindsay’s actions, irrespective of whether she denied or celebrated them. If she then married a radical jihadi, her status as the widow of a martyr would extend to him in terror circles. There are witnesses to the fact that she was having talks with radicals to discover if potential husbands were radical enough for her to marry.

Wishart arrived in Nairobi the Sunday after the Westgate attacks on 21 September last year. “I am pretty clear that she was never the mastermind of Westgate. And she probably didn’t have anything to do with it. At the time, they said she was inside, and people saw a woman with a gun, which subsequently proved to be bullshit. No woman was found. There were probably only four attackers. But you know, she was on the front pages.” And in this, she has the eternal appeal to the media of the absent, silent woman – just the possibility of her involvement turns the chaos of events into something like manageable current affairs, especially since it cannot, at least within the news cycle, be disproven.

It’s interesting to do a thought experiment about Westgate, with Lewthwaite masterminding it and without. With her, it feels closer to home, more relevant, a puzzle that might one day be solved. Without her, it feels wildly unpredictable, further away but more dangerous. And perhaps this is the reason she keeps cropping up in this guise as the mastermind, the ultimate bad guy; for the semblance of order … never mind the ins and outs of this terror organisation, at least we know the person at the top of it.

Usually, when nobody from the past will talk about a notorious wanted criminal, it’s because they were loners; scores of people will say they didn’t know them very well, nobody has any detail. In Lewthwaite’s case, she had many friends, none of whom wanted to talk, as Wishart describes: “It’s not clear what she’s done. There’s a real confusion about who she was, this best friend that they had. And how could she have gone in these wily ways? And then they have this doubt, which is, did they know her when Germaine was preparing 7/7? He killed more than anybody else that day – I think there’s a kind of guilt. I met a friend on a doorstep, and the first thing she said was that she couldn’t speak, out of respect for the 7/7 victims. Could they have, in their interactions, done anything that would have prevented the path she ended up taking? Some of them seem to feel a level of uncertain and unjustified culpability.”

Yet, despite them all clamming up, or possibly because of it, the film-maker came away with a distinct view: “My impression of her as a teenager was that she was a kind of bolshy girl. People certainly knew who she was in the class. My impression also was that people liked her. They really liked her.”

The White Widow: Searching for Samantha, is on 2 July on BBC1, 10.35pm