MI5 launched urgent inquiries yesterday into whether it had missed warning signs about the danger posed by Manchester bomber Salman Abedi.
The probe comes as the investigation spread to the south coast of England with the arrest of a 23-year-old man in Shoreham-by-Sea, West Sussex.
Searches are also continuing in Manchester and Cheshire following last week’s atrocity, which left 22 dead, including eight children.
St Ann’s Square in Manchester was flooded with a sea of floral tributes from well-wishers yesterday.
A senior Whitehall source said Abedi was a “former subject of interest” to security services whose risk “remained subject to review.”
A number of people who knew Abedi, including family members, had reportedly warned authorities he was developing radical views, prompting concerns that signs of the threat he posed were missed.
And early this year, the FBI warned British security chiefs that the Libyan-born Islamist was planning an attack in Britain.
Home Secretary Amber Rudd has already confirmed Abedi had recently returned to Britain after a visit to Libya.
Former fighters of the anti-Gadaffi Libyan Islamic Fighting Group, which was funded by MI6 in the 1990s, have revealed that MI5 assisted them to return to Libya during the uprising against the former leader, where they would have encountered other extremist organisations such as Isis and gained training in a warzone.
Ms Rudd said: “The intelligence services are still collecting information about him and about the people around him.
“But I would not rush to conclusions.”
In the wake of the Manchester attack, it emerged that British counter-terror authorities were dealing with 500 investigations into 3,000 individuals.