The security services are facing their worst crisis since the 2005 London bombing, with the increased frequency of attacks placing enormous pressure on the police and the UK’s domestic intelligence agency, MI5.
Questions are being raised about why the police and MI5 failed to prevent three attacks in quick succession.
MI5 will have to explain why it failed to monitor more closely one of the London Bridge attackers, Khuram Butt, given his links to al-Muhajiroun, the banned extremist group whose leader, Anjem Choudary, has been linked to the recruitment of more than 100 British terror suspects.
Officials will also face scrutiny for failing to act on at least two reports about Butt’s behaviour, one to the police from a neighbour who feared Butt was trying to radicalise children, and another to anti-terror hotline claiming he had been watching clips of a hate preacher.
Sources close to the security services say monitoring all potential suspects, even high-risk ones, is an impossible challenge and it is inevitable they will not be able to stop all plots.
MI5 has a staff of 4,000, with up to 1,000 more promised by 2020, to keep tabs on a list of 3,000 people classified as “subjects of interest”, who included Butt, and to engage in other activities.
Counter-terrorism accounts for just over 60% of what MI5 does.
The list of 3,000 can be seen as a pyramid, with only a tiny proportion at the top placed under 24-hour surveillance if suspected of planning an imminent attack.
Such surveillance is labour intensive, requiring two dozen or more officers, which limits the number of targets that can be closely monitored at any given time.
The rest of the pyramid encompasses a wide range of people, some associated with extremists and viewed as a medium risk, posing no imminent threat to life.
Another of the London attackers, Rachid Redouane, was not known to the police or MI5, the police said.
Richard Barrett, who worked for MI5 and was director of global counter-terrorism operations at the overseas agency, MI6, said it was not possible to monitor everyone under suspicion, and he would not want to live in a state like that.
He said it did not seem to him as if the security services had lost control, pointing out that while there had been three attacks since March, another five alleged plots had been foiled in the same period.
He said it was difficult to prevent attacks that involved knives.
MI5 is still heavily engaged in investigating the Manchester bombing and has still to close the investigation into the Westminster attack by Khalid Masood.
The security agencies have been digging deep into Masood’s Libyan connections and his internet communications.
MI5 will conduct later an internal review covering all three attacks, to see what lessons can be learned and whether it missed opportunities to prevent them.
No connection between the three attacks has yet been established.
A motive has still to be established for the London Bridge attack, including whether there was a copycat element.
There was scepticism over a claim of responsibility by Islamic State.
And Butt’s link to al-Muhajiroun is not regarded by itself as an explanation for Saturday’s attack.
Raffaello Pantucci, the author of We Love Death As You Love Life: Britain’s Suburban Terrorists, published two years ago, in which he blamed al-Muhajiroun for about half of British terrorist plots over the previous two decades, expressed sympathy for the security services
“The are clearly dealing with a substantial threat picture,” he said.
As well as the list of 3,000, there was a further list of 23,000 people who had been subjects of interests but had dropped off the main list.
The list of 23,000 continued to grow, with people only deleted once they had died, he said.
What was concerning, Pantucci said, was that there had been a perception in the UK that the threat had been limited to lone actors.
That had been undermined by the last two attacks, he said: the Manchester attacker, Salman Abedi, must have had someone teach him how to make the bomb, and the London attack involved three people.
Responding to a report that Theresa May was frustrated by the failure of MI5 to stop the spate of attacks, a source with high-level experience of counter-terrorism work said: “I hope she realises she will never regain the trust of important people such as MI5 and the upper echelons of counter policing.”
The source added of the counter-terrorism network involving MI5 and police: “It’s a game of fragile choices that have to be made.
You can double the resources and you will still have to make choices.
“If the numbers willing to do this escalates sharply, it is going to put a strain on the system. The system is not foolproof.”