Spy agencies are to use computer algorithms to keep tabs on 20,000 former terror suspects because they don’t have the manpower to physically watch all of them at once.

MI5 and MI6 will use the complex system to sift through massive amounts of data to alert intelligence agents to worrying behaviour, under potential plans.

A review is expected to recommend security agencies widen their searches when examining the online footprint and movement of these former jihadis.

Britain’s counter-terrorism officials currently actively monitor around 3,000 people.

But following a wave of terror attacks in the UK, the Government admitted there were as many as 20,000 former subjects of interest no longer classed as posing serious danger.

Those in this pool currently have little or no resources spent on them.

It is understood Khalid Masood, who killed five people in a terror attack in Westminster in March had previously come to the attention of counter-terrorism officers.

Salman Abedi, who bombed Manchester Arena, killing 22 people including young children, in May, and Khuram Butt, who attacked London Bridge in June, were both known to security agencies as well.

And the two terrorists who murdered Lee Rigby in 2013 were also in the pool of former ‘subjects of interest’. But none of them was under active investigation because it was believed they did not pose an immediate risk.

There are currently several internal reviews into the way MI5 and police handled the four terrorist attacks which occurred in the UK between March and June this year.

The fourth attack saw a van driver crash into pedestrians in London’s Finsbury Park. One man died at the scene.

David Anderson QC has been tasked with providing independent assurance of the internal reviews and is due to publish a report with findings and recommendations in a couple of weeks.

The official reviews are expected to clear MI5 and the police of making serious mistakes that allowed terrorists to strike Britain.

It is understood they will find information was received by MI5 about Abedi, which could have been given a higher priority – but that such a conclusion is reached with the benefit of hindsight.

According to The Guardian, nothing in the reviews demonstrate that clear chances were missed to stop any of the attacks that killed 36 people.

Last night Whitehall sources denied suggestions the report would be a whitewash and stressed that is why there was independent oversight.

One source close to the reports said: ‘Any suggestion security agencies are marking their own homework would be false.’

It is also understood Mr Anderson’s findings will suggest spy agencies expand the bulk collection of personal data to the list of 20,000 former terror suspects.

Sources said the findings would also set out the speed at which technology is changing and how MI5, MI6 and GCHQ must keep a pace with evolving threats. Intelligence officers are looking at ways they can spot signs that tens of thousands of citizens once high on their radar may be re-engaging with terrorism.

This will include using computers to collate mass amounts of information on a person, such as contact with other extremists on the internet.

One example could be using a hi-tech system to spot former suspects chatting in extremist groups online, or using information gathered from Oyster cards to spot a pattern and examine their whereabouts.

Parliament’s Intelligence and Security Committee is expected to decide today whether to carry out its own review of the Government’s handling of the terror attacks.

This article was originally published in The Daily Mail, written by Larisa Brown. View our archive of articles from The Daily Mail, here.

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