Manchester bomber Salman Abedi was a "former subject of interest" to British security services whose risk "remained subject to review", a government source says.
Details of the intelligence agencies' knowledge of Abedi came as police hunting the network behind his attack said they had made "significant" arrests and seized "very important" items in raids linked to the investigation of the blast at Manchester Arena that killed 22.
The senior source revealed that 18 plots had been foiled since 2013 in Britain.
Those included five since the Westminster attack in March this year.
It is understood the scale of the threat being dealt with by counter-terrorism agencies is on an "unprecedented" scale.
The source said at any one time there could be 3000 subjects of interest to MI5 and Abedi, 22, was part of a "larger pool" of people formerly classed in that category.
"MI5 is managing around 500 active investigations, involving some 3000 subjects of interest (SOIs) at any one time," they said.
"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."
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