We trust our security services to act as a shield in the face of terror. It’s their job to spot the danger and stop it long before it reaches an Arena of teenage girls.
So how did Salman Abedi slip through the net? Why didn’t they join the dots? And what led a man, born in Manchester, to become radicalised in the first place?
These are some of the questions today's report into the Manchester Arena Inquiry will seek to answer.
A total of 172,000 documents have been examined, and 291 witnesses and experts have given evidence in an inquiry lasting 196 days.
We’ve already heard about the security lapses which allowed Salman Abedi to wander through a train station with a bomb strapped to his back, and about the breakdown in communication which resulted in such a poor emergency response once that bomb had been detonated.
But for many the failings likely to be outlined tomorrow are the biggest of them all because had M15 intervened, Salman Abedi’s evil plan might have been thwarted long before that fateful night.
Who was Salman Abedi?
Salman Abedi grew up in Fallowfield in South Manchester - an area sometimes dubbed 'Little Tripoli' due to its large Libyan population.
His parents fled Libya after becoming opponents of Colonel Gaddafi's regime.
They had links to Didsbury mosque, which has been accused of not doing more to eradicate extremism.
The mosque says it had nothing to do with Abedi’s 'barbaric act', but whether they had a role in his radicalisation will come under scrutiny in tomorrow’s report.
Abedi's father, Ramadan Abedi, allegedly had links with the Libyan Islamic Fighting Group and the report is likely to point to an individual born into a world of islamist extremism.
In 2015, his elder brother Ismail was stopped on the way home from his honeymoon and found to have extremist material on his electronic devices.
But the only member of the Abedi family ever charged with any offence is Salman’s younger brother Hashem who was jailed for life in 2020 for his part in the attack.
And it’s a source of huge frustration for the bereaved that Salman Abedi’s family, who are all believed to be in Libya, have refused to co-operate with this inquiry.
Was Abedi known to MI5?
Salman Abedi was known to the security services. He first came to their attention in 2010 and was subject of interest in 2014 and 2015 but he had been taken off their watch list.
A meeting was meant to take place nine days after the attack to reassess the bomber. But by then, of course, it was too late.
We know he was in contact, either directly or indirectly, with eight different 'subjects of interest' - including individuals with links to al-Qaeda, Isis and groups in Libya.
We also know he visited a convicted terrorist, Abdalraouf Abdallah, in prison and that he spent time fighting in Libya with his brothers and father.
He flew back from Libya four days before the attack, so why wasn’t he stopped then?
MI5 say Abedi was one of 20,000 closed subjects of interest. In short, they can’t watch everyone.
But if they had watched him more closely, would the attack still have happened? And have lessons been learnt?
In addition, MI5 received two pieces of intelligence prior to the attack - the significance of which was not fully appreciated at the time.
MI5 wrongly linked them to criminal activity rather than terrorist activity and failed to pass this intelligence to the police.
However, we will never know what that intelligence was because three weeks of court hearings regarding their contents were held in secret due to concerns about national security.
MI5 stands by its decisions and says it couldn’t have pre-empted Salman Abedi’s plot, but the secrecy surrounding their evidence has angered many of the families who feel they had a right to hear all the facts.
Although tomorrow’s report is meant to bring them the truth, they will have to accept that they’ll never know the whole story.
The first inquiry report from retired high court judge Sir John Saunders, published in June 2021, focused on security arrangements at the venue.
It highlighted a string of “missed opportunities” to identify Abedi as a threat before he walked across the City Room foyer and detonated his shrapnel-laden device.
Sir John’s second report in November 2022 delivered scathing criticism of the emergency services response to the bombing.
He ruled that care worker John Atkinson, 28, would probably have survived but for the failures on the night, while there was a “remote possibility” the youngest victim, eight-year-old Saffie-Rose Roussos, could have lived with different treatment and care.
Sir John’s findings and recommendations on radicalisation and preventability will be published at 2pm on the inquiry website.
In the latest episode of From the North we ask should suicide prevention be taught in schools?