Third London attacker was Italian national of Moroccan descent
The third London Bridge terrorist has been named as Youssef Zaghba, an Italian national of Moroccan descent.
Scotland Yard confirmed the identity of the 22-year-old, from east London, and said he was not a police or MI5 "subject of interest", despite Italian media reports suggesting he was.
The news came as detectives arrested another suspect at an address in Barking, east London. The 27-year-old was held under the Terrorism Act shortly after 8am on Tuesday.
According to the Corriere della Sera newspaper Zaghba's Italian mother lives in Bologna in the north of the country.
The paper said Zaghba was born in Fez in Morocco and had seasonal work in a London restaurant. It reported that he was stopped by Italian police in March 2016 at Bologna's airport trying to fly to Turkey and then on to Syria.
Italian intelligence officials reportedly communicated this to their UK counterparts.
The name was confirmed as MI5 and police faced questions after it emerged another of the three perpetrators, Khuram Shazad Butt, 27, had been investigated two years before the atrocity.
Pakistani-born British citizen Butt and Rachid Redouane, who claimed to be Moroccan-Libyan, were named on Monday night as two of the terrorists who launched a murderous rampage around London Bridge and Borough Market on Saturday night.
Seven victims were killed and dozens injured in the spree, which ended when armed police shot dead the perpetrators just eight minutes after the first emergency call.
On Tuesday the Metropolitan Police's counter-terrorism command released the name and photograph of the third attacker.
Scotland Yard said: "While formal identification is yet to take place, detectives believe he is 22-year-old Youssef Zaghba, from east London.
"The deceased's family have been informed. He is believed to be an Italian national of Moroccan descent. He was not a police or MI5 subject of interest."
The latest arrest takes the total number held as part of the investigation to 13. Twelve have been released without charge.
A search warrant is being executed at an address in Barking, the Met said.
Earlier on Tuesday officers entered and searched an address in Ilford. Nobody has been detained at that location.
Butt was investigated by officers in 2015 but they found no evidence he was planning an attack and he was "prioritised in the lower echelons of our investigative work", Britain's top counter-terrorism officer said.
The disclosure means that perpetrators in all three of the terrorist outrages to hit Britain this year had at some point appeared on the radar of security agencies - although none was considered high risk at the time of the attacks.
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. In my view, we need to review what happened in his case, and learn the lessons so that the methodology of the response to known suspicions is improved."
Foreign Secretary Boris Johnson acknowledged the police and MI5 will face questions.
He told Sky News: "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? What happened? How can he possibly be on a Channel 4 programme and then committing atrocities like this?', and that is a question that will need to be answered by MI5, by the police, as the investigation goes on.
"I can't answer that question now."
Home Secretary Amber Rudd said it was not for her or Mr Johnson to "say how that happened or what happened".
She told BBC Radio 4's Woman's Hour: "It's like any incident - you will look and find out whether there is something to learn.
"I can say with absolute confidence that they will want to do that, but I can say that without, I hope, putting any sort of suggestion of blame because it's very easy to rush in and say 'What went wrong?'"
Metropolitan Police Assistant Commissioner Mark Rowley said police and their partners are "doing everything we can across the country to help prevent further attacks and protect the public from harm".
He said: "At any one time MI5 and police are conducting around 500 active investigations, involving 3,000 subjects of interest.
"Additionally, there are around 20,000 individuals who are former subjects of interest, whose risk remains subject to review by MI5 and its partners."
Security agencies and police have foiled 18 plots since 2013, including five since the Westminster attack.
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.
Transport for London (TfL) confirmed that Butt worked for London Underground for just under six months as a trainee customer services assistant, leaving in October last year.
The father-of-two, who reportedly appeared on Channel 4 documentary The Jihadis Next Door, was also reported to the anti-terror hotline in 2015 for showing signs of "extremism or radicalisation".
Security sources in Ireland confirmed that Redouane, 30, married a British woman in Dublin in 2012 and lived in the Rathmines area of the city. He also used the name Rachid Elkhdar, and claimed to be six years younger.
It is not clear when he entered Ireland or how long he stayed but it is believed he used Irish jurisdiction to get a European Union permit which allowed him to be in the UK.
An Irish security source described the murderer as having "extensive immigration history related to the UK".
Both Butt and Redouane lived in Barking, east London, but it is not yet known how the two men knew each other - with work to understand more about the trio and their connections under way.
The three knifemen ploughed into pedestrians on London Bridge with a van and went on to stab people in Borough Market with 12in (30cm) knives.
The attackers, wearing fake suicide vests, were shot dead by eight officers.
Thirty-six people remain in London hospitals, with 18 in critical care.
A minute's silence was observed at 11am on Tuesday in memory of the victims.
Londoner James McMullan, 32, and 30-year-old Canadian Christine Archibald were the first victims killed in the attack to be named.
Australian nurse Kirsty Boden was the third person to named among the dead.
An unnamed French national also died in the rampage, with two others missing, French foreign affairs minister Jean-Yves Le Drian said.
Scotland Yard said one person is still believed to be missing, and that family liaison officers have been deployed to support the victims' loved ones.
The Islamic State terror group has claimed the London Bridge rampage, while the attack has sparked fears that Britain is in the grip of a spate of copycat incidents.