The scale of the challenge facing the police and security services was disclosed by British government sources after criticism that multiple opportunities to stop the Manchester bomber had been missed.
According to The Times, about 3,000 people from the total group are judged to pose a threat and are under investigation or active monitoring in 500 operations being run by police and intelligence services. The 20,000 others have featured in previous inquiries and are categorized as posing a “residual risk”.
Security sources say the two terrorists who carried out attacks in Britain in 2017-Salman Abedi, the Manchester suicide bomber who killed 22 people, and Khalid Masood, the London killer-were from the pool of “former subjects of interest.”
Masood, who rammed a vehicle into pedestrians on London’s Westminster Bridge and then stabbed a police officer to death in March, was also in the pool. Both men were no longer under MI5 surveillance.
Early May, British media reports indicated that hundreds of ISIS terrorists are returning to the country from Syria, with the country's security forces saying they are unable to control the returnees.
Sources say the influx of ISIS is stretching the UK's security services to breaking point, with up to 30 officers required to provide 24-hour monitoring of just one suspect.
A few days before the Manchester attack last Monday, the UN Security Council's counterterrorism agency had warned that a wave of “dangerous and disillusioned” ISIS terrorists fleeing defeat in Syria and Iraq earlier this year might hit targets in Europe.
On Friday Britain’s opposition leader connected terrorist attacks at home with Britain’s failed wars abroad.
Addressing a press conference in London, Corbyn called for a change in how the UK operates in the world and handles domestic security.
He reiterated his longstanding view that engaging in wars like those in Iraq and Libya increase the threat of terrorist acts occurring at home.
