The third London Bridge attacker has been named as 22-yeard-old Youssef Zaghba, a Moroccan-Italian man.
He was stopped last year attempting to fly to Syria where he had planned to join ISIS.
Zaghba was detained at Bologna airport, as he tried to board a plane to Turkey.
ISIS propaganda films were reportedly found on his phone.
It is understood that he remained free after an investigation by the Italian authorities found insufficient evidence to prosecute him.
But warnings were reportedly given to both the British and Moroccan authorities.
Pakistan-born Khuram Butt, 27, and Rachid Radouane, 30, both from Barking were the other two attackers.
Meanwhile, another victim has been named as Australian Kirsty Boden, who her family said ran towards London Bridge to help people.
Seven people were killed and 48 injured in the attack which began at 10pm on 3 June.
NHS England said 36 people remained in hospital, with 18 in a critical condition.
Ms Boden’s family described her in a statement as an “outgoing, kind and generous person”.
“We are so proud of Kirsty’s brave actions which demonstrate how selfless, caring and heroic she was, not only on that night, but throughout all of her life,” they added.
Butt and his two accomplices drove a hired van into pedestrians on London Bridge before stabbing people in the area around Borough Market.
All three men were shot dead by police within eight minutes of receiving a 999 call.
All 12-people arrested on 4 June after the attack have now been released without charge. However, the Metropolitan Police said a 27-year-old man has been arrested in Barking on today 6 June regarding the incident.
A property in Ilford, east London, was also searched by police at around 01.30am, but no arrests had been made, the Met said.