An investigation into one of the London Bridge attackers was suspended because of an “unprecedented” threat level which put pressure on MI5 resources.
A senior MI5 officer told an inquest that the inquiry into Khuram Butt was suspended between March and May 2017.
Butt then killed eight people during the attack on 3 June 2017, which he carried out with two other men.
The court also heard that MI5 got an anonymous tip-off that Butt was “an extremist” more than two years before.
Butt, 27, Rachid Redouane, 30, and Youssef Zaghba, 22, mowed down pedestrians on London Bridge before launching a knife attack in nearby Borough Market, injuring 48 others.
Police shot and killed the attackers less than 10 minutes after the violence began.
Giving evidence shielded from public view, the officer – identified as Witness L – said the decision to suspend a number of investigations in 2017 was made amid “the unprecedented level of threat which we were facing and therefore the pressure on our resources”.
In March 2017 five people were killed during an attack on Westminster Bridge, and two months later 22 people died during an attack at Manchester Arena.
The head of policy, strategy and capability in MI5’s international counter-terrorism branch went on: “In my 28 years (in MI5), I cannot recall a time as alarming as this time.”
The investigation into Butt had also been suspended for around a month in February 2016 after a series of attacks in mainland Europe.
Witness L said: “I think it reflects the level of resourcing available. This and other similarly concerning investigations had to be suspended because there were even more concerning investigations above these.”
He added: “Money is not the key determinant here.
“Even if we’d asked for more money in November 2015, its ability to transfer into actual experienced investigators by 2016 would simply not be plausible.”
The Old Bailey heard that the security service received an anonymous call about Butt, more than two years before he carried out the attack, from an informant who specifically asked not to be contacted again.
Witness L said the call “identified an individual called Khuram Butt who was in the right sort of age range and said that he was an extremist”.
MI5 already had Butt on its radar – he was identified as a supporter of banned extremist group Al-Muhajiroun in 2014 – but did not realise the call related to the same person until after the attack.
This information was handed over before MI5 launched an official investigation into Butt in mid-2015 because of unconnected intelligence.
Investigation ‘well run’
The inquest has previously heard that Butt’s brother-in-law had reported him to a police anti-terror hotline in September 2015 – although this information was never passed on to the joint MI5 and police investigation of Butt.
Witness L told the court that a post-attack review, carried out by a manager in MI5 with a panel of experts, found “the investigation into Khuram Butt was well and effectively run”.
It also said the decisions to temporarily suspend investigations into Butt were “logical and proportionate in the circumstances”.
Police were not consulted about the suspensions “in any systematic way” but may have been spoken to informally, he added.
Xavier Thomas, 45, Christine Archibald, 30, Sara Zelenak, 21, Sebastien Belanger, 36, James McMullan, 32, Kirsty Boden, 28, Alexandre Pigeard, 26, and Ignacio Echeverria, 39, were all killed in the attack.
The inquest continues.