Youssef Zaghba, the third man identified as one of the authors of Saturday's deadly attack in London, was intercepted past year en route to Syria and signalled to Britain as a possible militant, Italian authorities said Tuesday.
"A 30-year-old man was arrested on suspicion of the commission, preparation or instigation of terrorist acts (contrary to section 5 Terrorism Act 2006) and has been taken into custody at a south London police station under the Terrorism Act 2000".
The three, who were wearing fake suicide vests, were shot dead late Saturday after ramming a van into pedestrians on London Bridge and then slashing and stabbing people in nearby Borough Market.
Despite a rapid response to Saturday's deadly attack, which saw armed officers shoot dead all three suspects within eight minutes, opposition parties have called for May's resignation for her cuts to police numbers.
The third attacker involved in the recent attacks in London city has been identified as Youssef Zaghba, the police confirmed on Tuesday.
Butt and Redouane, who also used the name Rachid Elkhdar, lived in Barking, while Zaghba is reported to have worked in a restaurant in London. All three have been claimed by the Islamic State group.
Zaghba, who was born in Morocco and lived in Italy, was stopped by border guards in Bologna in March 2016 as he tried to reach Syria via Turkey and join ISIS.
Foreign Secretary Boris Johnson acknowledged that the security services had to provide an answer.
They suggested that Butt was not "radicalised" in Pakistan, and that the family does not have links to religious parties. "He just kept saying, I have to do this for my family".More news: Doctor in Trump Hotel case released but due back in court
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He said Zaghba was always tracked by Italian intelligence officers while in the country and that United Kingdom authorities were informed.
The arrest comes as security services face growing questions over their monitoring of the three attackers in the lead up to the attack.
The BBC reports Zaghba was not prosecuted but was placed on the watch list.
Mohammed Shafiq, chief executive of the Ramadhan Foundation, said that after confronting Anjem Choudary for his support for terrorism near the Houses of Parliament a day after Lee Rigby's killing he was called a "traitor" by the future London attacker, Butt.
He was being monitored by Italian police and intelligence, sources said.
Two men were arrested by Irish police over documentation related to Redouane, one of whom has been released without charge.
Redouane married a British citizen in Ireland in 2012 and was not under surveillance, Irish prime minister Enda Kenny said.
Collina said she understood many British imams want to send a strong signal and have said they are not prepared to give her son a Muslim funeral.