MI5 chose not to investigate Westminster Bridge attacker Khalid Masood despite him having contact with a number of terrorists including a key figure in a major plot.
The extent of Mr Masood’s connection with various suspects being monitored by the security service was laid out at the inquests into the attack victim’s deaths on Wednesday.
Connections included one of the fertiliser bomb plotters and members of banned group al-Muhajiroun.
A senior MI5 officer gave evidence amid tight security at the Old Bailey, shielded from the sight of everyone in Court One by green curtains fastened together with bulldog clips.
He told the court that the decision in 2010 to class Mr Masood as someone who did not pose a risk to national security was “sound”.
Identified as Witness L, he said that the atrocity could not have been stopped.
He said: "There simply wasn't enough intelligence for us to work on that would have allowed us to identify his plot and work with the police to frustrate it.”
Because of strict security measures, no one other than lawyers and court transcription staff were allowed to use phones and computers during his evidence.
‘Engaged in attack planning on his own’
Witness L told the Old Bailey: “Masood engaged in attack planning on his own.
"It's very difficult to make a decision as to when he decided to mount the attack."
The court heard how Mr Masood came to the attention of MI5 in 2004, when his number was found in the phone of Waheed Mahmood, one of a group of terrorists who plotted to plant fertiliser bombs.
He also had contact on a number of occasions between 2004 and 2009 with another terrorist suspect under investigation by MI5.
The suspect had his address, email addresses and phone numbers.
In February 2010 there was a "vague and uncorroborated" report of someone called Khalid Masood, "an extremist based in Saudi Arabia", helping a UK terror cell plan to travel to Pakistan to receive training from al Qaida.
As part of the investigation into the cell, Mr Masood was initially classed as someone who posed a threat to national security, but then was downgraded to someone who may pose a threat the following month, when MI5 investigators discovered he was not involved in the plan.
In December that year a review found that as Masood was not involved in facilitating travel or in any other aspect of the plot, he could be downgraded again.
Witness L told the Old Bailey: "He was downgraded to a target who was not considered to be a risk to national security."
No record was made at the time of the reasons why the decision was taken.
‘Not everything can be stopped’
Counsel to the inquest Jonathan Hough QC read from a written statement by Witness L that said: "We have reviewed the decision to close Mr Masood (as a person of interest) with the benefit of hindsight and we conclude the decision was sound."
The officer went on: "It was logical, necessary and proportionate to consider him a closure."
Between December 2010 and October 2012 Mr Masood was associated with a number of subjects of interest to MI5 and there was "an indication that he consumed extremist material", the court heard.
For the next four years he "appeared intermittently in connection with a number of other subjects of interest", including some linked to banned extremist group al-Muhajiroun, but at no stage was there considered to be enough information to justify investigating him further.
The officer said that even though Mr Masood had a history of violent offending, had been linked to multiple terror suspects, and in 2013 expressed "satisfaction" that the September 11 attacks had drawn people to Islam, the decision not to investigate him was "a sound one".
Witness L said: "None of these indicators were enough to say that he was involved in activities of significant concern to us."
At the time of the attack in 2017 the service was facing an "unprecedented" scale of work, with around 500 investigations into Islamist terrorists, 3,000 people considered subjects of interest (SOIs) and 20,000 who had previously been classed as SOIs.
Mr Hough read Witness L a conclusion from a report by former terror watchdog David Anderson after the atrocity that said: "Not everything can be stopped, there will always be a danger of a determined attacker getting through."
The officer replied: "I'm afraid that's true."