British police ran a security check on Youssef Zaghba - the third London attacker - in January when he passed through London's Stansted Airport, according to two Italian security officials, raising questions about the assertion made by British authorities that he was not a "subject of interest" to United Kingdom security services.
According to the reports, the British and Moroccan secret services were notified of Zaghba's status as a potential militant. The company was looking into Italian media reports that Zaghba also was a former employee.
The youngest of Saturday's attackers is said to have told Italian authorities "I'm going to be a terrorist", while officers reportedly found Islamic State-related material on his mobile phone when they intercepted him.
Questions are being asked, in particular, about why nothing was done to stop Khuram Butt despite the fact he was reported to the police on two separate occasions for expressing approval of terrorist attacks and accessing online preaching and attempting to indoctrinate children.
Seven people were killed and 48 injured in the attack which began at 21:58 BST on Saturday night, June 3.
As the row threatened to overshadow the final days of the general election, Theresa May was facing questions over the actions of the intelligence agencies and the Home Office at a time when she was home secretary.
Questions over the extent to which the terror gang were known to security services have been mounting since it was revealed another of the attackers, Khuram Shazad Butt, 27, had been investigated in 2015.
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A Moroccan-Italian dual national, it has emerged he had tried to join the Islamic State group in Syria past year and was notified to Britain as a suspect figure after being intercepted by Italian authorities.
The Scotland Yard said one of them was Khuram Shazad Butt, 27, a Pakistan-born individual who had been investigated by MI5 and United Kingdom police.
London police said all 12 people held since the attack late Saturday from the Barking neighborhood in the east of the city had been freed without charge. Police have made 25 arrests for hate crime offences since the attack. "People are going to look at the front pages today and they are going to say, 'How on earth could we have let this guy or possibly more through the net?"
All three attacks have been claimed by the Islamic State group. Yet, while confirming that Zaghba was one of the killers, Scotland Yard stated: "He was not a police or MI5 subject of interest". Like Butt and Redouane, he lived in east London.
The prosecutor said Italy had lacked the evidence to prosecute Zaghba, but had warned Britain about him.
But police said that they had no warning of a plot and that their investigation of Butt had been shelved.
He was investigated in 2015 but he was "prioritised in the lower echelons of our investigative work" and there was no evidence of "attack planning", police said.
Butt was previously known to security agencies and had appeared in a British TV documentary called "The Jihadis Next Door".
All three men were shot dead at the scene of the attack by officers within eight minutes of police receiving the first emergency call. Eighteen people remain in critical condition. The attacker was then "neutralised" by police, he added. Between 2009 and 2016, the number of police officers fell by nearly 20,000, or around 14 percent, according to the independent Institute for Fiscal Studies think tank.