Salman Abedi may have built the bomb that killed 22 people at the Manchester Arena in under four days, it has emerged amid reports he met Isis-linked militants in Libya.

Police said the attacker purchased parts for the bomb after flying back from the country on 18 May, while experts confirmed terror training would have enabled Abedi to build it in 24 hours.

The 22-year-old had made numerous trips to Libya after his parents returned to live there during the country’s bloody civil war, possibly fighting against Muammar Gaddafi’s soldiers alongside his father.

​Islamist groups swiftly gained power in the conflict, where Isis has seized the opportunity to gain a foothold amid warnings the country could become a primary launch pad for terror attacks in Europe.

Among the countless militias battling in Libya is Katibat al-Battar al-Libi, an Isis special operations unit whose operatives are said to have met Abedi during his visits to Tripoli and the coastal city of Sabratha.

A retired European intelligence officer told the New York Times that Abedi kept in contact with the militants after returning to Manchester.

A CCTV image showing Salman Abedi between 18 and 22 May, before he carried out the Manchester attack (Greater Manchester Police)

“When Abedi was in the UK, the contacts happened sometimes by phone,” he said. “If the content of the call was sensitive, he used phones that were disposable, or dispatches were sent from Libya by his contacts to his ‘friend’ - living in Germany or Belgium - who then sent it to Abedi in the UK.”

British intelligence agencies are working with their Libyan counterparts to piece together Abedi’s activities in the country, which is split between rival governments and thousands of militias.

​Katibat al-Battar al-Libi was originally formed by Libyan jihadis fighting in Syria in 2012, and attracted Belgian, French and Tunisian foreign fighters.

Among them was the Paris attacks ringleader Abdelhamid Abaaoud, who was also linked to terror attacks at the Brussels Jewish Museum, the Thalys train attack and several failed plots.

Research by Cameron Colquhoun, the managing director of corporate intelligence consultancy Neon Century, found that many of the Battar brigade’s members moved back to Libya and set up terror training camps, focusing on mass murder, weapons training and bomb-making.

Among the terrorists to have gone through training by affiliated fighters in Sabratha is the Isis gunman who killed 30 British tourists on a beach in Tunisia in 2015.

US warplanes target ISIL in Libya for first time

Isis training camps used by “plotters actively planning operations against Europe” on the coast have been targeted by US air strikes, which have killed commanders.

The first concrete link between Libya and terror attacks in Europe came in December, when a failed Tunisian asylum seeker rammed a lorry into a Christmas market in Berlin.

Investigators found that the attacker, Anis Amri, had been communicating with Isis fighters in the group’s Libyan stronghold of Sirte using an encrypted messaging app.

Europol has warned that as the so-called Islamic State loses swathes of territory in Syria and Iraq, it will adopt new tactics to attack the West as defeated jihadis return home to Europe or move into other conflict zones.

“Libya could develop into a second springboard for Isis, after Syria, for attacks in the EU and the North African region,” said a report.

“Since mid-2015 Libya has become a major destination for Isis fighters in its own right and is believed to having become a hub for EU foreign terrorist fighters who, on returning to Europe, plan further terrorist attacks.”

Europol warned that jihadis in the country might have been driven out of Sirte but still possess stockpiles of weapons and “unlimited places in which jihadists could be trained for future terrorist attacks”.

A piece of metal with Arabic writing and the Isis flag, after US air strikes hit a terror training camp west of Sabratha, Libya in 2016 (AP)

If Abedi underwent terror training in Libya – or in Syria as some officials have suggested – he is likely to have been instructed on bomb-making.

Greater Manchester Police have said he made “core purchases” for the device detonated at Manchester Arena alone, and in the four days between when he flew back from Libya and launched the attack.

Like those built by Syria-trained Isis militant Najim Laachraoui for the Paris and Brussels attacks, the Manchester bomb used the homemade explosive triacetone triperoxide (TATP).

It has become a hallmark of Isis attacks and plots, being found mid-manufacture by supporters in Germany and France, and can be made cheaply from commercially-available chemicals.

Dr Sidney Alford, an explosives engineer, said making a lethal quantity of the explosive could take just 24 hours.

“It’s simple,” he told The Independent. “It takes only about a couple of hours to make, then you need to filter it and wash it and dry it.

“How long it takes to dry depends on your facilities but in a normal house with a radiator or something to stand it on, you could leave it overnight.”

Dr Alford, the chairman of Alford Technologies, said TATP is volatile and activated extremely easily, describing it as “sensitive”.

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. 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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. 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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

“I’m quite sure that a person who has been on a terrorist course will be instructed by people who will know how to arrange things so that the possibility of killing yourself [during manufacture] is quite low,” he added.

“I’m guessing that Abedi was given experience in doing it.”

He is believed to have packed the explosive in powder form inside a bag, surrounded by screws and nuts intended to inflict maximum death and injury.

Dr Alford said evidence from the scene of the blast suggested Abedi had a device to activate the bomb himself, but that there may also have been a secondary detonator capable of receiving a radio signal from a phone anywhere in the world.

The possibility would chime with Isis propaganda statements issued shortly after the attack, which claimed that a device had been remotely detonated and did not describe the atrocity as a suicide bombing.

Greater Manchester Police said Abedi made many movements alone in the four days between arriving back in the UK and launching the attack, moving around Manchester with a blue suitcase they are trying to trace.

A “significant” car was discovered in the Rusholme area, where he made several visits in his final days, was discovered on Friday, with a 24-year-old man being arrested.

Greater Manchester Police said he is one of 11 men, aged between 18 and 44, who remain in custody on suspicion of terror offences.

Officers have been stepping up security in the city ahead of a fundraising concert by Ariana Grande, who has visited fans injured in the attack in hospital.