MI5 may have missed the chance to connect two of the London Bridge attackers before they struck because of admin errors, a senior officer said.
Youssef Zaghba was stopped at Bologna airport in 2016 after telling staff he was going to Turkey to be a terrorist.
Italian authorities asked Britain for more information on him but MI6 sent the request to the wrong person in MI5 and it was never acted on.
Zaghba and two accomplices killed eight people in the attacks on 3 June 2017.
They were shot dead by police after driving a van into pedestrians on London Bridge and stabbing people in and around Borough Market.
Forty-eight people were also injured.
The senior MI5 officer – identified as Witness L – was giving evidence at the Old Bailey at the inquests into the victims’ deaths.
The court heard how Zaghba was stopped at the airport in March 2016, , on his way to Istanbul.
Asked why he was going to Turkey, he said to be “a terrorist” before quickly changing his answer to “tourist”, the court heard.
The Italian authorities put him on a Europe-wide serious crime watch list. However, MI5 were not aware of this because he had not been flagged as a cross-border national security risk.
The month after Zaghba was stopped at the airport, Italian officials asked the UK’s overseas spy agency, MI6, a series of questions relating to him.
Witness L, who is head of policy, strategy and capability for MI5’s international counter-terrorism branch, told the court MI6 did not translate the Italian request for two months – and then sent it to the wrong person in MI5.
Giving evidence shielded from public view, Witness L told the court that individual did not file or act on the memo.
“I suspect this was probably a misunderstanding,” he said.
“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,” he added.
The inquest heard previously that MI5 had been investigating another attacker, Khuram Butt, since 2015, but did not learn of his association with Zaghba until after the attack.
Witness L accepted it was “possible” the administrative errors in Zaghba’s case had denied MI5 an opportunity to link him with Butt.
He said had it received the Italian request, it would most likely have replied asking for more details about Zaghba – but that it was “unlikely” to have launched an active investigation into him.
He said: “Flagging as a person of interest, particularly as they came in and out of the UK border, feels more likely.”
The lawyer representing six of the victims’ families had previously told the court there had been missed opportunities to prevent the attack.
Xavier Thomas, 45, Chrissy Archibald, 30, Sara Zelenak, 21, Sebastien Belanger, 36, James McMullan, 32, Kirsty Boden, 28, Alexandre Pigeard, 26, and Ignacio Echeverria, 39, were all killed by the trio.
The inquests continue.