To his distraught mother, Sudesh Amman was a “polite, kind, lovely boy” who became radicalised while serving time at the high-security Belmarsh prison, which houses a number of terrorism convicts.

Yet to the judge who jailed him in 2018, Amman was already someone displaying strong and often extreme views whose “life goal” was to die a “martyr” and be admitted to paradise.

And to the armed undercover officers who were surveilling him after his release from prison less than two weeks ago, he was an immediate and real threat to life – a threat they were forced to eliminate on a street in front of panicked families and Sunday shoppers.

No terrorism offenders to be released without review, says minister Read more

Amman’s journey from north London science student to lone jihadist bringing terror to Streatham High Road will be scrutinised by the relevant authorities. The key question he bequeaths in death is how society deals with any future Ammans.

His mother, Haleema Faraz Khan, 41, originally from Sri Lanka, told journalists that when she first heard of Sunday’s attack, she “had a feeling” Amman, 20, the eldest of her six children, was responsible because it was in south London.

He had been staying at a hostel in the area since his automatic release from prison on 23 January. He had served half of a 40-month sentence, imposed at the Old Bailey in December 2018, when he pleaded guilty to possessing and distributing terrorist material.

It was there that his mother visited him on Thursday, and she spoke again to her son by phone on Sunday. Just a few hours later, he grabbed a knife from a convenience store and launched his random stabbing spree, injuring two people, one seriously. A third person was injured by flying glass during the police gunfire.

“I spoke to him on the phone on Sunday. He said: ‘Mum, I want some biryani ... your mutton biryani,” Khan told Sky News in tears.

“He was fine when I went to see him. He became more religious inside prison, that’s where I think he became radicalised. He was watching and listening to things online which brainwashed him. Before he went to prison he was not that religious. After he came out he was really religious.”

She added: “He was a polite, kind, lovely boy. He was always smiling. I’m so upset, he was only 20 years old.”

It has been reported that concerns were raised about him in prison, but evidence shows Amman was radicalised before his time at Belmarsh.

There was no sign of remorse when he appeared in the Old Bailey dock, smiling as the sentence was passed and even waving to his mother and brother in the public gallery.

He was first flagged to counter-terrorism by a Dutch blogger who focused on exposing online extremism and in April 2018 noticed Amman was active on the Telegram messaging app.

Amman’s posts included an image of a knife, along with two firearms on a black Shahada flag, and the Arabic words meaning “armed and ready, April 3”. He had linked it to a YouTube video of a pro-gay rights protester, due to be at Speakers’ Corner, Hyde Park, and urged others to “all unite together and attack one another”.

When police arrested Amman, they found a stash of terrorist material, including manuals on bomb-making, knife fighting and close combat. They also seized an airgun, a black flag and a combat knife.

He had shared an al-Qaida magazine in a family WhatsApp group, including images of poses reminiscent of Islamic State-inspired supporters, and focusing on the idea of the “lone jihad”.

He had sent beheading videos to his girlfriend, and urged her to kill her “kuffar” (non-believer) parents. In messages to her, he pledged his allegiance to Isis, and his wish to carry out acid attacks, his trial heard.

Elsewhere, he wrote that he preferred the idea of a knife attack over bombs, and discussed whether he would stand his ground if police came to arrest him. In one notebook, in which he had made notes on explosives and detonators, he wrote of his “goals in life”. These included “die as a shuhada” (martyr) and go to “Jannah” (paradise).

Jailing him after his subsequent guilty plea, Judge Mark Lucraft QC said Amman’s interest in Islamic extremism and Isis “appears to be more than just an immature fascination”. Much of his fascination with conducting an attack was focused on using a knife, but reference was also made to committing acid attacks on mopeds, he said. It was clear, he added, that Amman was “someone with sincerely held and concerning ideological beliefs”.

Amman had originally been arrested on suspicion of planning a terrorist attack, but in the event was charged only in relation to ownership and distribution of terrorist propaganda and instruction manuals.

After his sentencing, just over one year ago, acting commander Alexis Boon, then head of the Metropolitan police counter-terrorism command, said: “It is not clear how Amman became radicalised but it is apparent from his messages that it had been at least a year in development.”

At the time of his 2018 arrest, Amman was living in Harrow, north London, with his mother and younger siblings in a rented house. He had moved from Park high school in Stanmore and was studying maths and science at the College of North West London. The college said in a statement that he was a student there from September 2017 to May 2018 “during which time there were no indications that he was potentially linked to terrorist activities”.

According to his mother, his ambition was to study biomedical science. He had one previous conviction, for possession of an offensive weapon – a bottle – and cannabis.

A former neighbour in Harrow, Anthony, said the father had been absent in recent years, and lived abroad, though the family did go on trips to see him.

Another neighbour described them as a “very nice” family, sympathising with the mother who she said was “raising the children on her own”.

Facebook Twitter Pinterest Police officers conduct a search of a bail hostel in south London after the Streatham attack. Photograph: Daniel Leal-Olivas/AFP via Getty Images

Anthony, however, claimed police had been called to the house on at least two occasions in recent years. “He [Amman] was actually a nice kid, he wasn’t a bad kid,” he recalled, saying he had lived on the street for his whole life.

“They [the family] were noisy, always trouble in there. The house was smashed up, there were fights inside,” he said.

“I used to see him go to the mosque [wearing traditional Islamic dress] … Before that he was all right and he started going there and he kind of changed, you could see it in him.”

He continued: “He was actually an intelligent kid. He did science and maths. Then he dropped out. I used to see him all the time where he was in and out, talk to him and stuff.”

A teenage girl who did not wish to be named said she knew Amman from the neighbourhood. He used to talk about being a terrorist, she said, but she and others thought he was joking.

A former schoolfriend spoke of his regret at the life Amman had chosen, telling Sky News: “It really is a shame that he has done such a tragic thing, as I believe that Sudesh really had potential to make it far in life.”

Police were conducting detailed forensic searches of properties linked to Amman, including the hostel. Its manager, who did not wish to be named, said: “I didn’t have much to do with him.

“The last time I saw him he was doing his radiators, setting up his heating on Friday.”