Up to 23,000 people have emerged on the radar of British counter-terror agencies, according to figures released Friday by U.K. security forces.

British authorities revealed that 500 investigations into 3,000 individuals were ongoing in the wake of Monday’s terror attack in Manchester. Security sources claim that another 20,000 people have been considered “subjects of interest” at some point in time, the Press Association reported Saturday.

Britain’s domestic counter-intelligence and security agency MI5 considered Salman Abedi, the suicide bomber in the attack, a “former subject of interest” who was “subject to review.” Despite being flagged to authorities on at least five separate occasions, they did not regard Abedi as an imminent threat.

Khalid Masood, the man who killed five people in the March Westminster attack, was in the same category of people who are no longer subject to any surveillance due to a lack of resources.

“All those people are in the mix and they have to be looked at,” Security Minister Ben Wallace told BBC Radio 4’s Today Programme Friday, according to the Press Association. “All of that is predominately underpinned by intelligence … Unfortunately the hardest part is we’ve got to convert intelligence into evidence if we actually want to deprive people of their liberty or take certain steps.”

The British parliament announced plans to reform its prison system last year, as the country’s maximum facilities have turned into “jihadi training camps.” The “most subversive extremist prisoners” will be kept away from the general population to reduce the risk of radicalization.

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