Suicide bomber Salman Abedi is believed to have travelled to Syria and become radicalised before returning to the UK to cause carnage at a gig in the city where he was born.
The son of Libyan parents, who reportedly fled their native country and sought refuge in the UK, he is thought to have come back to Britain from Libya just days before the massacre.
Here's everything we know about the 22-year-old Manchester Arena attacker.
Born and raised in Manchester in 1994, Abedi, the second youngest of four children, grew up in a Muslim household but matured into a university dropout with an appetite for bloodshed.
His parents, mother Samia Tabbal and father Ramadan Abedi, a security officer, are Libyan-born refugees who fled to the UK to escape Gaddafi. It is thought they returned in 2011 following Gaddafi’s overthrow.
Abedi is thought to have an older brother Ismail Abedi, who was born in Westminster in 1993, a younger brother Hashim Abedi, and a sister Jomana, whose Facebook profile suggests she is from Tripoli and lives in Manchester.
A family friend, who asked not to be named, said the family were known to the Libyan community in the city and described Abedi as "normal".
He told the Press Association: "He was always friendly, nothing to suggest (he was violent). He was normal, to be honest."
Abedi is believed to have attended the Manchester Islamic Centre, also known as the Didsbury Mosque.
Sheikh Mohammad Saeed said he believed Abedi had displayed a "face of hate" after the imam gave a sermon denouncing terrorism.
Abedi went to Burnage Academy for Boys between 2009 and 2011, and then on to Salford University in 2014 where he studied business management before dropping out, according to a source.
The source said Abedi began his course in 2014 and attended lectures for two years but then stopped going. He would have graduated this summer.
He did not live in university accommodation, had not been in any trouble at the university and was not on any radar for pastoral or social care.
It is understood Abedi was not known to have participated in any clubs or societies during his time in higher education and never met with the resident imam.
He was registered as living at the Abedi family home Elsmore Road, south Manchester as recently as last year, where plainclothes police raided a downstairs red-bricked semi-detached property on Tuesday.
Neighbours recalled an abrasive, tall, skinny young man who was little known in the neighbourhood, and often seen in traditional Islamic clothing.
He is thought to have lived at a number of addresses in the area, including one in Wilbraham Road, where officers arrested a man on Tuesday.
Abedi previously lived with his parents and a brother.
Neighbours in Elsmore Road told how Abedi had become increasingly devout and withdrawn.
Lina Ahmed, 21, said: “They are a Libyan family and they have been acting strangely. A couple of months ago he [Salman] was chanting the first kalma [Islamic prayer] really loudly in the street. He was chanting in Arabic.
“He was saying ‘There is only one God and the prophet Mohammed is his messenger’.’
A family friend, who described the Abedis as “very religious”, said most of the family had returned to Libya, leaving only Salman and his older brother Ismail behind.
“They have not been there for quite a while. Different people come and go,” said Alan Kinsey, 52, a car-delivery driver who lives across the street. Mr Kinsey’s wife, Frances, 48, a care worker, said she believed that the parents had left before Christmas and just one or two young men had been living in the property.
Mr Kinsey said a huge flag, possibly Iraqi or Libyan, had been hanging from their house. “There was a large Iraqi flag hanging out the window but we never thought anything or it,” added Mr Kinsey, “We thought it was about football or a protest at home or something.”
Abedi has "proven" links with Islamic State, according to France's interior minister.
Gerard Collomb told French television that both British and French intelligence services had information that Abedi had been in Syria.
Mr Collomb said: "All of a sudden he travelled to Libya and then most likely to Syria, became radicalised and decided to commit this attack."
Abedi had been a "regular kid", who went out and drank until around a year ago when he "dropped off the radar", the Times reported the bomber's former landlord's nephew as saying.
The paper quoted a friend as saying he had returned from a three-week trip to Libya in recent days.
Abedi's trips to Libya are now subject to scrutiny including links to jihadists.
A group of Gaddafi dissidents, who were members of the outlawed Libyan Islamic Fighting Group (LIFG), lived within close proximity to Abedi in Whalley Range.
Among them was Abd al-Baset Azzouz, a father-of-four from Manchester, who left Britain to run a terrorist network in Libya overseen by Ayman al-Zawahiri, Osama bin Laden’s successor as leader of al-Qaeda.
Azzouz, 48, an expert bomb-maker, was accused of running an al-Qaeda network in eastern Libya. The Telegraph reported in 2014 that Azzouz had 200 to 300 militants under his control and was an expert in bomb-making.
It has emerged in US media reports that the bomber was identified by a bank card in his pocket.
According to NBC News, citing a US intelligence official, members of the bomber's family warned security officials about him in the past, saying that he was “dangerous”.
The official told the broadcaster that Abedi likely "had help" making the “big and sophisticated bomb".
Speaking for the first time about his son's death, Abedi's father said: "We don't believe in killing innocents. This is not us."
Speaking from Tripoli, he told AP this his son was innocent and confirmed that British police had arrested another of his sons, believed to be a 23-year-old arrested in south Manchester on Tuesday.
Abedi's sister, Jomana, suggested he carried out the attack for revenge on US air strikes in Syria.
“I think he saw children—Muslim children—dying everywhere, and wanted revenge," she told the Wall Street Journal. "He saw the explosives America drops on children in Syria, and he wanted revenge. Whether he got that is between him and God.”
His brother Hashim reportedly knew he was planning the attack.
"His brother felt there was something going on there in Manchester and he thought his brother would do something like bombing or attack. So after that, he told us, 'Having internet, I see the attack in Manchester and I knew that's my brother'," a spokesman for Libyan authorities told BBC2's Newsnight.
He revealed that Abedi's younger brother Hashim had been investigated for about a month and a half over suspicions that he was linked to IS.
"We were not quite sure about this, but when we arrested and we asked him, he told us, 'I have ideology with my brother'. Hashim told us, 'I know everything about my brother, what he was doing there in Manchester'."
Abedi is not the first radical associated with the North West.
It has been reported he knew fellow Mancunian Raphael Hostey, who was once described as an "inspirational figure" for would-be jihadis.
Hostey left the UK in 2013 and became known as Abu Qaqa in his capacity as an IS fixer, encouraging other young Britons to join the terror group. He was believed to have been killed in a drone strike in 2016.
Libyan refugee Abdalraouf Abdallah was sentenced to five-and-a-half years in jail last year after helping Muslim convert and former RAF Iraq War veteran Stephen Gray try to get to Syria.
Abdallah, wheelchair-bound after he was shot in Libya in 2011, lived in Westerling Way, Moss Side, a short drive from Abedi's Elsmore Road address.
Gray, who lived at nearby Whitnall Street in the city, was jailed for five years for terror offences, after he twice attempted to join jihadis in Syria.
Jamal al-Harith, who lived in Manchester and was known as Ronald Fiddler before converting to Islam, left the UK for Syria in 2014.
Earlier this year it was reported that he died after driving a truck packed with explosives into a military base in Mosul, Iraq.
It emerged he had received a compensation payment following his detention in Guantanamo Bay in the early 2000s.