There has been a sharp increase in the number of terrorist plots foiled by UK security agencies since the Westminster attack in March, a senior Whitehall source has said.
Five plots have been disrupted in the last two months alone, compared with 13 in the three years between 2013 and March 2017.
At any one time MI5 is juggling around 500 active investigations relating to 3,000 people of interest, according to a source.
Five planned terror attacks have been prevented since the Westminster attack on 22 March – a sharp and disturbing increase on the average in recent years of one foiled plot every three months.
The workload facing security officials trying to prevent attacks is now said to be “unprecedented”.
The source also confirmed that Manchester bomber Salman Abedi was on security services’ radar as a “former subject of interest”.
The 22-year-old, who killed 22 people and injured 64 when he blew himself up at the end of an Ariana Grande concert on Monday, was “one of a larger pool of former subjects of interest whose risk remained subject to review” by security agencies.
“MI5 is managing around 500 active investigations, involving some 3,000 subjects of interest (SOIs) at any one time”, the source told the Press Association.
“Abedi was one of a larger pool of former SOIs whose risk remained subject to review by MI5 and its partners.
“Where former SOIs show sufficient risk of re-engaging in terrorism, MI5 can consider re-opening the investigation, but this process inevitably relies on difficult professional judgements based on partial information.”
It comes as Greater Manchester Chief Constable Ian Hopkins said detectives had made “significant” developments in their investigation.
“I want to reassure people that the arrests that we have made are significant, and initial searches of premises have revealed items that we believe are very important to the investigation”, he said.
The days since Monday’s attack have been “intense” for police staff, he added.
A total of eight people have now been arrested in the UK, in addition to Abedi’s father, Ramadan, and younger brother, Hashem, in Libya. A woman who was arrested on Wednesday has since been released.
Police carried out raids in Manchester, Wigan and Nuneaton on Thursday and are carrying out searches at a number of properties.
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