MI5 is expected to come under fire this week for missing a series of chances to prevent the Manchester Arena terror attack.
Suicide bomber Salman Abedi killed 22 people when he detonated his home-made device at an Ariana Grande concert in May 2017.
The final report from the public inquiry into the atrocity is out on Thursday.
Sir John Saunders, who led the probe, has already highlighted failings by the emergency services.
Now he is expected to criticise MI5’s failure to act on intelligence.
Much evidence about what MI5 knew about Abedi was heard behind closed doors at the inquiry, to safeguard national security.
But it is known he had been flagged on three separate occasions over fears he was showing signs of radicalisation.
He was declared a “subject of interest” in March 2014, which meant he was placed under investigation by MI5, but the file was closed four months later.
It was reopened in October that same year but this time closed after one day.
In the months before May 2017, MI5 twice received intelligence on Abedi but labelled it “non-terrorist criminality”.
The Security Service also knew Abedi was in regular contact with a convicted terrorist in early 2017.
But he was not questioned on his return to Britain from Libya on May 18, 2017, four days before killing himself in the explosion aged 22.
His younger brother Hashem, now 24, got life in 2020 for aiding the plot.
Their brother Ismail, 29, left the UK in 2021.
MI5 lawyers have said decisions on Abedi were in the context of “unprecedented” scale of terrorist threat in 2017.
Victims’ families’ lawyer Richard Scorer said: “We hope Sir John analyses... all the various failings and makes suitably wide-ranging recommendations.”
