The meeting was to boost cooperation on terrorism and migration in the first meeting of a new forum, the European Relationship for Mediterranean Security.
The 27-year-old man is the only suspect now in custody.
And according to Italian media reports, Zaghba's status as a potential militant had been notified to the British and Moroccan secret services.
He was stopped at Bologna's airport trying to fly to Turkey in March previous year amid concern he was intending to travel on to Syria, according to reports.
Italian intelligence officials reportedly communicated this to their United Kingdom counterparts.
In Britain's third Islamist attack in as many months, Butt and two others rammed a van into pedestrians on London Bridge on Saturday evening before running into the bustling Borough Market area where they slit throats and stabbed people.
French President Emmanuel Macron said Wednesday that a third French victim has been identified among those killed in the London Bridge van-and-stabbing attack in London. The deceased's family have been informed. He was believed to be living in east London and is described as an Italian national of Moroccan descent.
Redouane's wife is among those under arrest in London as part of the investigation into Saturday night's atrocity. Police arrested several people and are widening their investigation after a ser. The search in Ilford, just north of Barking, is seeking to determine whether the group had accomplices. Nobody has been detained at that location.
According to reports in the Italian newspaper Corriere della Serra, he had been recently working in a restaurant in London. He was still considered a risk, the paper alleged.
Former anti-terror laws watchdog Lord Carlile told the Press Association: "I feel a sense of disappointment this morning that the perpetrator Butt slipped off the radar".
Foreign Secretary Boris Johnson acknowledged authorities will face questions.
He married a British woman named Charisse O'Leary, who posted on Facebook last month that Redouane was negligent in seeing their young daughter and on one planned visit, she said he told her: "I'm going swimming". She said she would rather not answer that question, and tears began to roll down her cheeks.
The family said her actions "demonstrate how selfless, caring and heroic she was, not only on that night, but throughout all of her life".
Police said they had to prioritize resources on suspects who were believed to be preparing an attack or providing active support for one. He has said cuts in the number of police officers have had an impact on the ability to prevent attacks.
Butt, who is thought to have also gone by the name Abu Zaitun, was known to neighbours by the nickname "Abs/z" and was allegedly an associate of radical hate preacher Anjem Choudary.
In fact, Butt was one of the main characters in a television documentary called The Jihadis Next Door.
"Our Common Travel Area with Britain is one that has existed for 90 years, giving the right of travel, residency, business and to avail of social-welfare benefits, so these are issues that obviously will be reflected upon in the days to come", he replied. His name had also been flagged in 2015 by F.B.I. informant Jesse Morton who told authorities he had met Butt in an extremist internet chatroom used by Anjem Choudary, one of Europe's most prolific hate preachers who was jailed a year ago for his support of the Islamic State group, according to a British government official who spoke on condition of anonymity because he was not authorized to speak about the ongoing attack investigation.