Britain's intelligence agencies have warned ministers of a potential new wave of terrorist attacks carried out by "bedroom radicals" bred during lockdown.
Officials believe the country is facing a new threat from "lone wolf" terrorists who were radicalised online while spending months at home, The Telegraph understands.
It comes as investigators seek to establish whether the 25-year-old suspect in the killing of Sir David Amess had been radicalised during the long months of the pandemic.
One security source said: "Counter-terror police and MI5 have been concerned for some time that once we emerged out of lockdown there would be more people out on the streets and more targets for the terrorists.
"Combined with the fact that lots of young people have been spending so much time online, it makes for a very worrying mix and there is a real concern about the possible rise of the bedroom radicals."
On Sunday night police were continuing to question 25-year-old Ali Harbi Ali, a British national of Somali heritage, on suspicion of murdering Sir David, the Tory MP, in a knife attack on Friday.
Over the weekend it emerged that Mr Ali had previously been referred to the Government's counter-extremism programme, Prevent, but was not on MI5's terror watchlist.
It has raised fresh questions over the effectiveness of Prevent, which is currently the subject of an ongoing review.
Investigators now believe the suspect may have considered killing other MPs, the Telegraph understands.
Police believe Sir David was not specifically targeted by his alleged terrorist killer but is believed to have been picked as part of plot to murder any MP.
Counter-terror police had initially thought the Right-wing Tory politician had been selected because of his values, views or religion. But it is now feared he was stabbed to death simply because he happened to respond to his alleged killer's request for a face-to-face appointment as part of an indiscriminate attack on democracy.
It is understood the suspect had booked an appointment to see the MP at his regular Friday surgery a week before the attack.
The development came as Sir David's devastated family issued a statement in which they said they were "absolutely broken" by his murder.
They called for unity rather than division in the wake of his "cruel and violent" death.
The statement read: "We ask people to set aside their differences and show kindness and love to all. This is the only way forward. Set aside hatred and work towards togetherness.
"Whatever one's race, religious or political beliefs, be tolerant and try to understand."
It was reported on Sunday night it was believed Ali was “radicalised” after watching extremist videos on YouTube and became “obsessed” with the hate preacher Anjem Choudary, sources told The Sun.
The Telegraph has learned that the suspect in custody was seen using his phone moments after the attack.
Police are said to be investigating whether he made a recording in order to take credit for the attack or was sending a message or posted material online after the fatal stabbing.
This alleged activity, which was seen by witnesses, is understood to be at the centre of why the police labelled the attack a "terrorist incident".
Investigators were over the weekend trying to work out who the suspect had been in contact with in the run-up to Friday to better understand his alleged motives.
Meanwhile, three properties in London, connected to the man in custody, were being searched, including his former family home in Croydon, where his mother and siblings still live, and a property in Bounds Green, where his father resides.
Detectives were also at a flat in Kentish Town in north London, where Mr Ali had been living most recently.
Priti Patel, the Home Secretary, spoke about concerns that Covid lockdowns may have fuelled radicalisation and the threat of lone wolf attacks.
Asked about the issue on BBC One's Andrew Marr show, Ms Patel said: "It's a really important point actually, it really is. Coronavirus, pandemic, people being locked down at home, online, etcetera.
"But what I would say, to put this into perspective, we have the best security and intelligence agencies in the world.
"I can't sit and share with you [but] I know how they worked throughout the pandemic. I know the work that they do in terms of watching individuals – subjects of interest, tracking behaviours, monitoring anybody of interest."
Security chiefs are understood to be concerned after months of lockdown left millions of younger people marooned at home, with many spending hours online, often unsupervised.
With schools, sports clubs and youth facilities also closed, there was little opportunity for the usual support networks to spot worrying signs of radicalisation and alert the authorities.
It is feared extremists around the world will now seek to activate their online recruits and encourage them to carry out terror strikes across the UK.
Focus on effectiveness of Prevent
The review of the Prevent programme has been running all year and is expected to be nearing its conclusion, though a publication date for conclusions is yet to be named.
William Shawcross, who chaired the Charity Commission between 2012 and 2018, is leading the work after being made the independent reviewer in January.
The review is said to be a top-to-bottom look at how the program is working, with Sir William tasked with writing a report for the Home Office that contains "recommendations on improvements".
Robert Buckland, the former justice secretary, used his first broadcast interview since being sacked from the role in the reshuffle last month to call for a more "joined up" approach to Prevent.
Mr Buckland told Times Radio: "I very much hope that when it comes to community supervision and community involvement with people like this particular individual, that it is much more joined-up between health services, education, whatever it might be, who have had some involvement with that individual in the past.
"And I think that that element of being joined-up is what we really need to work on urgently."
Asked how agencies could work more closely, Mr Buckland said: "There may be records or information from schools or colleges or from the health service which can tell us much more about individuals and their activities.
"I think we need to join this up much more effectively because what we're talking about here is community prevention.
"We've got to make sure that every arm of the state is absolutely working together in order to understand as much as possible about these individuals."
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