Video report by ITV News Correspondent Sejal Karia
A tip-off by Italian authorities that one of the London Bridge attackers was in the UK got lost in MI5’s filing system, an inquest has heard.
Youssef Zaghba, one of three attackers who collectively killed eight people in June 2017, told Italian authorities in 2016 he was travelling "to be a terrorist" before correcting himself to "tourist" - when trying to fly from Bologna to Istanbul.
A senior MI5 official, identified only as Witness L, told the inquest on Thursday that a request for intelligence from Italian authorities into attacker Youssef Zaghba had not been actioned.
When asked why MI5 took no action, he said: "Because the request, which was a request for tracing of the type we discussed the other day, went to an incorrect addressee."
Giving evidence shielded from public view, the officer, who is head of policy, strategy and capability in MI5’s international counter-terrorism branch, added: "It went to the wrong MI5 addressee.
"I can find nowhere it was filed within the MI5 records."
He said: "I suspect this was probably a misunderstanding.
"I suspect the individual to whom it was sent did not understand they needed to take any action at all.
"No response was given to the Italian authorities as far as I am aware."
But when asked if the "administrative errors" that occurred in connection with the Italian authorities’ request had deprived security services of the opportunity of linking Zaghba to fellow attacker Khuram Butt, Witness L said: "Yes, it is possible."
He added: "So I think this is an interesting and inevitably speculative chain of events.
"Had we been interested, I suspect our first response would have been to return to the Italian authorities and to ask for more detail, because the information provided, namely that he’d arrived at an airport and said he was a terrorist, and then changed it to tourist, is unusual.
"So our first step would have been to ask the Italians for more information.
"Had they provided information of interest, I think it is possible we would consider putting him on a terrorist watch list in addition to the existing serious crime watch list."
But Witness L said an investigation into Zaghba would have been "unlikely" and added: "So I think active investigation would have been unlikely, but flagging as a person of interest, particularly as they came in and out of the UK border, feels more likely."
Butt, 27, Zaghba, 22, and Rachid Redouane, 30, killed eight people and injured 48 more in a van and knife attack on June 3 2017.
They mowed down pedestrians on London Bridge before stabbing innocent bystanders at random in nearby Borough Market.
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.
Investigators failed to identify the school where Butt and Zaghba both taught - the Ad-Deen primary school in Ilford - which was run by the wife of known extremist Sajeed Shahid.
Witness L had previously accepted that a potential investigative opportunity had been lost as well, with a failure to examine more closely the gym that Butt, Zaghba and Redouane all attended, again owned by Shahid.
When asked if a system could be introduced to flag up whether known suspects were renting vehicles Witness L said it was possible but would might require new legislation.
But he told the inquest that with about 15 million vehicles rented a year in the UK he would not want intelligence services having to look through all of them.
He added: "I don't think the principle difficulties should be underestimated.
"It is really important, and much work is being done to consider what more needs to be done in terms of vehicle hires."
Dominic Adamson, representing the parents of victim Xavier Thomas and his partner, said that despite two people raising concerns about Butt's extremism none of that information fed into any security assessments.
He added: "Two individuals who had identified his extremism, one who had certainly identified the fact that he was sharing extremist material, and yet neither of those reports, none of that information fed into any assessment that was ever made of Khuram Butt, was it?"
But while Witness L agreed the reports had not made it into any assessment of Butt he said the information they contained was already known to security services.
He added: "So I don't believe they are terribly important for they simply reflect material that was already known about Butt, but I would accept that they did not reach the appropriate teams in both cases."
When asked if this was a "significant failing" he said: "I would not share that view, given that the information really simply reinforced that which we already knew about Butt."
The MI5 officer told the inquest that there was no intelligence that Butt had taken up jobs in door security or at Transport for London for "nefarious purposes".
But Witness L said he was unaware of any arrangements that were in place for counter terrorism police to be notified of terror suspects applying for these kind of positions.
When asked if a terror suspect could work at Heathrow Airport or major railway stations he added: "So clearly both of those will have a number of vetted posts, but individuals who do not have a vetted post and who are potentially suspects could work at those locations, yes."
When asked if this was a risk he said: Yes, but that's the nature of the vetting system, whereby the individual airports and other places will make judgements as to how great the risks are."
The inquests continue.