Seven were killed and almost 50 wounded.
"One attack can be excusable, a second one maybe, but a third one, no - especially in the run-up to a general election". British security officials said none of the men was considered violent, but they acknowledged the difficulty of predicting whether extremists will turn unsafe.
The Italians claim both MI6 and MI5 were informed about Zaghba, who told police he was "going to be a terrorist" when he was stopped at Bologna airport. His cousin, 18-year-old Bilal Dar, told The Associated Press that Butt's uncle was taken in for questioning. "This has destroyed our family's pride".
Zaghba was reportedly detained briefly past year, while Butt appeared in a TV documentary focussing on Jihadis in the UK.
AFP has compiled what is known about the three men, amid an ongoing investigation into the third deadly jihadist assault in Britain claimed by the Islamic State group in less than three months.
During the attack, seven people were killed and dozens more were wounded.
The Italian passport also meant he could easily travel to and from Britain. They worked in the same restaurant: Kentucky Fried Chicken in London's East Ham area. He also worked at Kentucky Fried Chicken and used a gym in east London.
He said Zaghba was always tracked by Italian intelligence officers while in the country and that United Kingdom authorities were informed.
Authorities say the threat is unprecedented, with 500 active investigations involving 3,000 individuals in addition to 20,000 former subjects of interest.
The arrest comes as security services face growing questions over their monitoring of the three attackers in the lead up to the attack. "That's where the question of resources comes into play". The couple is thought to have split.
It had been set at "critical" in the days after the Manchester concert bombing on May 22 that killed 22 people - reflecting a judgment that an attack might be imminent because accomplices with similar bombs might be on the loose. Twelve people arrested earlier have since been released.
The film was a close encounter with part of the ALM network and one of its subjects was Siddhartha Dhar, one of Anjem Choudary's right-hand men.
Irish state broadcaster RTE reported on Tuesday that Redouane had been refused asylum in Britain but had been able to move legally after marrying in Ireland in 2012. British officials declined to comment on why his asylum request was denied.
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.
But in April 2016, the Bologna court stopped the investigation, cancelling the order under which Zaghba's phone had been seized and denying permission for police to retain a copy of his contacts. And according to Italian media reports, Zaghba's status as a potential militant had been notified to the British and Moroccan secret services.
"We did everything we could have done", he said.
"We did everything we could have done, but there weren't elements of proof that he was a terrorist", he added. "He was someone who was suspicious because of his way of behaving".
However, Italy put him on a list of people considered "at risk". Zaghba's Italian citizenship prevented such an expulsion, Italian daily Repubblica reported.
Ruscigno said "the only member of the family who lived here was the mother, who had not been seen for a while".
"Last year. when I went to England, he was a bit more rigid", Collina, a convert to Islam, told reporters in a series of interviews Wednesday.
"Undoubtedly when people see something which appears from their perverted point of view to be successful, some people will be inspired by that", she said.
As details about the jihadists have emerged, May has faced questions about her record of overseeing cuts to police numbers when she was interior minister.
Since 2010, when the Conservatives entered government, the number of police officers has fallen from 145,948 to 126,766 in March 2016, according to Home Office figures cited by ITV.