Youssef Zaghba, one of three authors of Saturday's deadly attack in london, was intercepted last year en route to Syria and flagged to Britain as a possible jihadist, Italian authorities said on Wednesday.
Zaghba, a 22-year-old with dual Moroccan-Italian nationality, was briefly detained after being stopped at the airport in Bologna, central Italy in March 2016.
Italian prosecutors were unable to bring charges against him for links to international terrorism and he was set free, a police spokesman told.
Italy notified Britain and Morocco of his status as a potential Wahhabi radical.
But he never showed up on Britain's radar as someone capable of involvement in an attack like Saturday's, in which seven people were killed.
The three attackers smashed a van into pedestrians on london Bridge before going on a stabbing spree which ended with them being shot dead by police.
london's Metropolitan Police said Zaghba had not been on their radar before Saturday, despite the Italian warning. He was not a “police or (intelligence agency) MI5 subject of interest,” a spokesman said.