The security services are facing questions over how they monitor potential threats after one of the men who took part in the London Bridge attack was revealed to be a known extremist.
Pakistani-born Khuram Shazad Butt was on the radar of the police and MI5 and had apparently been investigated in 2015.
Believed to ringleader of the London Bridge attack, Butt was known to have links to al-Muhajiroun, the banned extremist group led by radical hate preacher Anjem Choudary.
But the investigation into Butt was “prioritised in the lower echelons of our investigative work”, Britain’s top counter-terrorism officer Mark Rowley said.
Mr Rowley said that while Butt, who reportedly went by the name Abu Zaitun and was known to neighbours by the nickname “Abs/z”, was known to the security services, there was no evidence of “attack planning” by him.
The revelation also means that perpetrators in all three of the terrorist outrages to hit Britain this year had at some point appeared on the radar of authorities.
Also identified in the London Bridge attack was Rachid Redouane, who claimed to be of Moroccan and Libyan origin and sometime went by the name Rachid Elkhdar.
He was not known to authorities.
Security sources in Ireland confirmed Redouane, 30, had married a British woman in Dublin in 2012 and lived in the Rathmines area of the city.
It is not clear when he arrived Ireland or how long he stayed but it is believed he used Irish jurisdiction to get a EU permit which allowed him to be in the UK.
Police are working to identify the third attacker.
Both Butt and Redouane lived in Barking, east London, but it is not yet known how the two men knew each other.
All 12 people arrested on Sunday after the London attack have now been released without charge.
Meanwhile more than 130 imams and religious leaders announced their refuse to say funeral prayers for the London Bridge perpetrators, and called on others to do the same.