LONDON (Reuters) - British security services have thwarted 18 militant plots in the UK since 2013, including five following an attack in central London in March, a source with knowledge of the matter told Reuters.

“MI5 is managing around 500 active investigations, involving some 3,000 subjects of interest at any one time,” the source told Reuters on condition of anonymity.

MI5, established in 1909 to counter German espionage ahead of World War One, is tasked with protecting British national security. Most of MI5’s work is focused on counter-terrorism and it employs around 4,000 people.

MI5 is trying foil several plots against Britain at any one time, the source said, adding that the Manchester suicide bomber, Salman Abedi, had been on its radar.

“Abedi was one of a larger pool of former subjects of interest whose risk remained subject to review by MI5 and its partners,” the source said.

“Where former subjects of interest 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.”