British counter-terror agencies missed several chances to stop Salman Abedi before he carried out the Manchester Arena bombing, it has emerged.

Two people who knew the 22-year-old at college called the anti-terror hotline after he told them "being a suicide bomber was okay".

The calls are thought to have been made five years ago, after Abedi left school.

The brother of Salman Abedi, Ismail, poses for a picture with a machine gun

Two community leaders also reported Abedi over his extremist views.

Mohammed Shafiq, chief executive of the Ramadhan Foundation, told The Daily Telegraph Abedi was reported two years ago "because he thought he was involved in extremism and terrorism".

“People in the community expressed concerns about the way this man was behaving and reported it in the right way using the right channels," Mr Shafiq said.

“They did not hear anything since.”

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 was also reported to the authorities after he confronted an imam who was delivering an anti-extremist sermon at Didsbury Mosque.

He claimed the leader was “talking bollocks” during the speech condemning Isis, the Manchester Evening News reported.

Akran Ramadan, who is part of the Libyan community in south Manchester, said he understood Abedi had been placed on a "watch list" after the mosque banned him and reported him to the authorities for his extremist views.

The attacker's family had also reportedly warned authorities he was "dangerous".

Officials scouring the background of the British-born bomber said he was likely part of a wider terrorist network.

Armed soldiers have been deployed around the country following the Manchester attack (Carl Court/Getty Images)

Several arrests have been made in both Britain and Libya after the bombing, which killed 22 people and wounded 64 more.

Eight men have been taken into custody in the UK, while the bomber's father and younger brother have been arrested in Libya, the latter of whom confessed to knowing "all the details" of the terror plot.

Before becoming increasingly religious as his radicalisation deepened, Abedi drank and took drugs.

He had shown signs of extremism by flying black flag associated with Islamist groups from his Manchester home.

“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.

Flowers, messages and candles are pictured in St Ann's Square in Manchester in tribute to the victims of the terror attack (Getty)

The Times reported that Abedi was also 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.

British authorities were probing whether Abedi had ties to other cells across Europe and North Africa, two anonymous officials familiar with the case told the Associated Press.

They said one thread of the inquiry involved pursuing whether Abedi was part of a larger terror cell that included Mohamed Abrini, otherwise known as "the man in the hat," with connections to the Brussels and Paris attacks.

Abrini visited Manchester in 2015 and reportedly told police he took photos of Manchester football stadium.

Abedi is known to have returned from a visit to Libya just days before the Manchester attack, and it is feared he may have travelled to terrorist strongholds in Syria.

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

Ramadan Abedi, the father of Salman Abedi (Reuters)

Abedi's father has said his son is innocent.

Before his arrest, he said he had last spoken to his son five days ago and he sounded "normal."

He said his son was getting ready to visit Saudi Arabia for a short Umrah pilgrimage then planned to head to Libya to spend the Islamic holy month of Ramadan with his family.

He said his son last visited Libya about six weeks ago and had never gone to Syria. He denied ties to any militant groups or suggestions of extremism.