But police and intelligence officials came under intense scrutiny after it was revealed Butt had appeared in a Channel 4 documentary on British jihadis a year ago, and was known for his extreme views.
The other two attackers were named Monday as Khuram Shazad Butt and Rachid Redouane.
Rachid Redouane (center) and Youssef Zaghba have been named as the suspects in Saturday's attack at London Bridge.
A police auto then screeches into shot, with three armed policemen exiting and immediately trying to rescue the man by diverting his attackers' attention towards them.
Police have arrested at least 14 people in connection to the attack, though most have been released without charge.
The so-called Islamic State (IS) group has said its "fighters" carried out the attack.
Zaghba "was not a police or MI5 subject of interest", according to Metropolitan Police.
An Italian police source has said that Zaghba, who lived in east London, had been placed on a watch list, which is shared with many countries including the UK.
Last year, Zaghba was stopped at the airport in Bologna on his way to Istanbul and was put on an EU-wide database but was not prosecuted.
He previously claimed to be of Libyan and Moroccan descent - using multiple aliases, police said.
"The attack began at about 10 p.m., local time, when a van crossing London Bridge veered into pedestrians, crashing outside the Barrowboy & Banker Pub". The Guardian said he was a pastry chef who had lived in Dagenham in east London, not far from where Butt was residing.
Australian woman Kirsty Boden was among the seven killed.
Neighbors described Butt as an avid weightlifter and Transport for London confirmed he worked for London Underground in customer service before leaving last October.
Her family are reported to be distraught and "bracing for the worst".
He added: "We are paying a heavy cost in these attacks".
Delcros was badly injured in the attack and is now recovering in hospital after undergoing a two-hour operation to reset her shattered pelvis.
A French national was also killed in the attack, according to foreign minister Jean-Yves Le Drian. The men continued the assault in the adjacent Borough Market area, a popular bar and restaurant district.
Investigators confiscated Zaghba's computer but did not have proof of a crime so they had to return the device and pointed him out to London as a possible suspect, Amato said.