Salman Abedi’s journey from cannabis-smoking university dropout to Isis suicide bomber is being gradually pieced together by investigators as Manchester reels from the worst terror attack to hit the UK in over a decade.

The 22-year-old was remembered as a “fun guy” who drank, took drugs and possibly had links with local gangs before appearing increasingly religious as his radicalisation deepened.

The same transformation has been documented in many of Europe’s deadliest terrorists, including petty criminals and drug dealers in the Isis “super cell” behind the Paris and Brussels attacks.

A report by the International Centre for the Study of Radicalisation (ICSR) found that more than half of known European jihadis had criminal histories, making them easy fodder for Isis recruiters offering “redemption” in the name of jihad.

Professor Peter Neumann, director of the ICSR at King’s College London, said the emerging “crime-terror nexus” was making radicalisation harder to spot for security services.

“There is now a perfect fit between these young men and a group that has shed any attempt at serious theological discourse,” he told The Independent on the report's release.

“Criminals are already used to violence, so for the jump from being an extremist to being a violent extremist is much smaller.”

Police name Manchester attack suspect as Salman Abedi

Mr Neumann warned that the UK and other countries need to “rethink our strategy” to spot the warning signs for this new and dangerous form of jihadi.

Analysts said Isis propaganda portraying a life of action, adventure, brotherhood and purpose was particularly appealing for those facing dim prospects at home.

Research by the Combating Terrorism Centre additionally found the majority of Isis recruits had not always been observant Muslims, were unemployed or students and had an immigrant background – all factors shared by Abedi.

Investigators are probing his potential links with militants and recruiters from Isis, which claimed responsibility for his massacre in a statement hailing a “soldier of the caliphate”.

Abedi, who was born in the UK to Libyan parents who had fled Muammar Gaddafi’s regime, had shown signs of extremism including flying an Islamist flag from his Manchester home but not raised sufficient alarm to spark intervention by British security services.

The Burnage Academy for Boys confirmed he attended the school between 2009 and 2011 but said it could give no further details due to the ongoing investigation.

Abedi went on to study for a business and management degree at the University of Salford but dropped out at the start of his second year in September, getting a job in a bakery.

He was not well-known by neighbours in the tight-knit Fallowfield community in Manchester, but some noticed he had grown a beard and started to wear religious dress in recent months.

Police raid a house on Elsmore Rd in Fallowfield, where suspected suicide bomber Salman Abedi lived (Eamonn and James Clarke) (Eamonn and James Clarke)

A person who said they knew Abedi from school told the Manchester Evening News: “He was an outgoing fun guy but since he went to Libya in 2011 he came back a different guy.

“He used to drink, smoke weed then all of a sudden he turned religious and I’ve not seen him since 2012.”

Other clues pointing to radicalisation include the appearance of a black flag associated with Islamist groups on Abedi’s house in Elsmore road.

“There was a black flag with Arabic writing on it on the roof for a bit, a few years ago,” a neighbour told the BBC.

Leaders at the nearby Didsbury Central Mosque, where his father and brother are said to have prayed, told The Independent they were “shocked” and did not know Abedi.

Fawzi Haffar said: “We have many Libyans here but the sermons are extremely moderate, we are very liberal...the preaching and the way we teach Islam is the correct Islam, in our opinion.

“We have no hardcores as far as I know. We always make sure we are very mainstream.“

Abedi used a suitcase bomb packed with homemade explosives and nails for the attack – the same method used for Isis’ bombings at Brussels Airport and Molenbeek metro station last year.

Police officers arrive at a residential property on Elsmore Road in Fallowfield, Manchester, on May 24, 2017, (AFP/Getty Images)

CCTV footage reportedly showed the 22-year-old putting the suitcase down in the foyer between Manchester Arena and Victoria station shortly before it detonated amid Ariana Grande fans pouring out of the concert.

The Times reported that Abedi was known to security services as an associate of Isis recruiter Raphael Hostey, also from Manchester, who was killed in a drone strike in Syria last year.

Other prolific militants from the city include Abdalraouf Abdallah who was jailed last for attempting to help fellow jihadis reach Isis territories in Syria after being shot and paralysed during the Libyan uprising.

A family friend told The Independent Abedi and his brother, Ismael, remained in the UK when their parents returned to Libya with the rest of the family four years ago but travelled back and forth to the war-torn country.

He returned from his most recent trip just days before Monday’s attack, sparking suspicions he may have been acting as a “mule” carrying a suitcase bomb built by Isis militants.

"He went to Libya three weeks ago and came back recently, like days ago,” a friend told The Times.

Chaos engulfing Libya since the British-backed removal of Muammar Gaddafi in 2011 has left armed groups including Islamist factions battling for control as smugglers launch thousands of refugees from its shores.

Isis has gained a foothold in the country, using it for terror training camps bombed by the US in January over intelligence that “external plotters were actively planning operations against Europe”.

Manchester explosion in pictures Show all 37 1 /37 Manchester explosion in pictures Manchester explosion in pictures People running down stairs as they attempt to exit the Manchester Arena after a blast, where U.S. singer Ariana Grande had been performing, in Manchester Twitter/@ZACH_BRUCE/ via REUTERS Manchester explosion in pictures Helpers attend to people inside the Manchester Arena after a suspected suicide bomber detonated an explosive device at the end of an Ariana Grande concert, killing 22 people PA wire Manchester explosion in pictures Armed officers guard outside a hotel near the Manchester Arena following reports of an explosion, in Manchester, Britain EPA Manchester explosion in pictures Police officers are seen outside the Manchester Arena, where U.S. singer Ariana Grande had been performing, in Manchester, northern England Reuters Manchester explosion in pictures MANCHESTER, ENGLAND - MAY 23: Police escort members of the public from the Manchester Arena on May 23, 2017 in Manchester, England. An explosion occurred at Manchester Arena as concert goers were leaving the venue after Ariana Grande had performed. Greater Manchester Police have confirmed 19 fatalities and at least 50 injured. (Photo by Christopher Furlong/Getty Images) Getty Images Manchester explosion in pictures MANCHESTER, ENGLAND - Emergency services arrive close to the Manchester Arena on May 23, 2017 in Manchester, England. There have been reports of explosions at Manchester Arena where Ariana Grande had performed this evening. Greater Manchester Police have have confirmed there are fatalities and warned people to stay away from the area. (Photo by Dave Thompson/Getty Images) Getty Images Manchester explosion in pictures MANCHESTER, ENGLAND - MAY 23: Police escort members of the public from the Manchester Arena on May 23, 2017 in Manchester, England. An explosion occurred at Manchester Arena as concert goers were leaving the venue after Ariana Grande had performed. Greater Manchester Police have confirmed 19 fatalities and at least 50 injured. (Photo by Christopher Furlong/Getty Images) Getty Images Manchester explosion in pictures Medics deploy at the scene of a reported explosion during a concert in Manchester, England on May 23, 2017. British police said early May 23 there were "a number of confirmed fatalities" after reports of at least one explosion during a pop concert by US singer Ariana Grande. Ambulances were seen rushing to the Manchester Arena venue and police added in a statement that people should avoid the area / AFP PHOTO / Paul ELLIS (Photo credit should read PAUL ELLIS/AFP/Getty Images) AFP/Getty Images Manchester explosion in pictures Police and fans close to the Manchester Arena, after reports of an explosion Getty Images Manchester explosion in pictures There have been reports of explosions at Manchester Arena where Ariana Grande had performed Getty Images Manchester explosion in pictures Police deploy at scene of explosion in Manchester, England, at a concert in Manchester Arena AFP/Getty Images Manchester explosion in pictures Police stand by a cordoned off street close to the Manchester Arena Getty Images Manchester explosion in pictures Police deploy at scene of explosion in Manchester, England AFP/Getty Images Manchester explosion in pictures Police deploy at scene of a reported explosion during a concert in Manchester, England, on May 23, 2017. British police said early May 23 there were "a number of confirmed fatalities" after reports of at least one explosion during a pop concert by US singer Ariana Grande. Ambulances were seen rushing to the Manchester Arena venue and police added in a statement that people should avoid the area AFP/Getty Images Manchester explosion in pictures An ambulance drives away from the Manchester Arena, where U.S. singer Ariana Grande had been performing, in Manchester Reuters Manchester explosion in pictures Police escort members of the public from the Manchester Arena in Manchester, England. Getty Images Manchester explosion in pictures A woman sits in the street in a blanket near the Manchester Arena as police guard the area following reports of an explosion, in Manchester, Britain EPA Manchester explosion in pictures Two women wrapped in thermal blankets stand near the Manchester Arena, where US singer Ariana Grande had been performing, in Manchester Reuters Manchester explosion in pictures A Royal Logistic Corps (RLC) bomb disposal robot is unloaded outside the Manchester Arena following reports of an explosion, in Manchester. At least 19 people have been confirmed dead and others 50 were injured, authorities said. It is being treated as a terrorist incident until police know otherwise EPA Manchester explosion in pictures A Royal Logistic Corps (RLC) bomb disposal robot is unloaded outside the Manchester Arena following reports of an explosion, in Manchester EPA Manchester explosion in pictures Members of the public receive treatment from emergency service staff at Victoria Railway Station close to the Manchester Arena on May 23, 2017 in Manchester, England. There have been reports of explosions at Manchester Arena where Ariana Grande had performed this evening. Greater Manchester Police have have confirmed there are fatalities and warned people to stay away from the area Getty Images Manchester explosion in pictures Armed police after a suspected terrorist attack at the Manchester Arena at the end of a concert by US star Ariana Grande left 19 dead PA wire Manchester explosion in pictures Emergency services arrive close to the Manchester Arena in Manchester Getty Images Manchester explosion in pictures An amoured police vehicle patrols near Manchester Arena in Manchester Getty Images Manchester explosion in pictures A man carries a young girl on his shoulders near Victoria station in Manchester AFP/Getty Images Manchester explosion in pictures Police officers stand at the Miller Street and Corporation Street Crossroads, in front of the Manchester Arena in Manchester, England Getty Images Manchester explosion in pictures Police block a road near to the Manchester Arena in central Manchester, England AP Manchester explosion in pictures Armed police patrol near Victoria station in Manchester, northwest England. Twenty two people have been killed and dozens injured after a suspected suicide bomber targeted fans leaving a concert of US singer Ariana Grande in Manchester Getty Images Manchester explosion in pictures Police forensic officers leave the Manchester Arena as they investigate the scene of an explosion in Manchester Getty Images Manchester explosion in pictures A forensic officer collects evidence on a walkway between Victoria station and Manchester Arena following a deadly terror attack in Manchester, Getty Images Manchester explosion in pictures A woman and a young girl wearing a t-shirt of US singer Ariana Grande talks to police near Manchester Arena following a deadly terror attack in Manchester, Getty Manchester explosion in pictures epaselect epa05982725 Susan Walton and daughter Katie, 10 (pictured), who attended the concert of Ariana Grande at the Manchester Arena, are seen in Manchester, Britain, 23 May 2017. According to a statement released by the Greater Manchester Police on 23 May 2017, police responded to reports of an explosion at Manchester Arena on 22 May 2017 evening. At least 19 people have been confirmed dead and around 50 others were injured, authorities said. The happening is currently treated as a terrorist incident until police know otherwise. According to reports quoting witnesses, a mass evacuation was prompted after explosions were heard at the end of US singer Ariana Grande's concert in the arena. EPA/NIGEL RODDIS EPA Manchester explosion in pictures Mayor of Greater Manchester Andy Burnham and Manchester City Council Leader Sir Richard Leese speak to the media outside Manchester Town Hall after a suicide bomber killed 22 people, including children, as an explosion tore through fans leaving a pop concert in Manchester Dave Higgens/PA Wire Manchester explosion in pictures The media gather behind a police cordon in Manchester Getty Images Manchester explosion in pictures Flowers left close to the Manchester Arena, the morning after a suicide bomber killed 22 people, including children, as an explosion tore through fans leaving a pop concert in Manchester Danny Lawson/PA Wire Manchester explosion in pictures Ariana Grande concert attendees Karen Moore and her daughter Molly Steed, aged 14, from Derby, leave the Park Inn where they were given refuge after last night's explosion at Manchester Arena Getty Manchester explosion in pictures Signs saying 'We love Manchester' are displayed in a window in Manchester, England Getty Images

Abedi’s movements inside the country are unclear, but France’s new interior minister said he may have travelled onwards to Syria.

“All of a sudden he travelled to Libya and then most likely to Syria, became radicalised and decided to commit this attack,” Gerard Collomb told BFMTV.

Asked whether Abedi was part of a larger terror network, he replied: “We don't know yet. In any case, links with Daesh (Isis) are proven.”

Of at least 850 extremists known to have left the UK for Isis territories in Iraq and Syria, at least half are believed to have returned and 15 per cent been killed.

As the group loses swathes of territory Europol warned of an “increased rate in the return of foreign fighters” either to the EU or to other conflict zones including Libya.

They will include militants with the expertise to build devices like the nail bomb Abedi used to kill at least 22 people and injure more than 60 others.

As well as training an extensive network of bombmakers, Isis has periodically released manuals on how to manufacture homemade explosives using commonly available materials, as well as picking out targets including concerts for terror attacks.

Amber Rudd, the Home Secretary, confirmed Abedi had recently returned from Libya and said the nature of his attack suggested he may have had support.

“It was more sophisticated than some of the horrific events that we have seen in the past or in other parts of Europe so people are reasonably wondering whether he did this on his own,” she added.