Britain’s most senior police officer has said the Streatham attacker was able to stab people despite being under surveillance because such operations are not “man-to-man marking”.
Met Police commissioner Cressida Dick told a committee that Sudesh Amman was under “covert” police surveillance.
Amman, 20, was shot dead by police after stabbing a teacher, 51, and a man in his 40s in south London on Sunday.
Dame Cressida said it was “clearly not possible” to stop every attack.
Amman had been released from prison on 23 January after serving half of his sentence for terror offences.
He was under active police surveillance at the time of the attack, which police believe to be an Islamist-related terrorist incident – he had a hoax device strapped to his body.
On Wednesday Dame Cressida gave evidence to the London Assembly Police and Crime Committee about general tactics used by counter-terror police.
She said when officers put people under surveillance “they are not of course providing man-to-man marking”.
“They are there covertly and that is a deliberate thing,” she said.
She added: “I wish I could assure the public that everybody who poses a risk on the streets could be subject to some sort of thing that would stop them being able to stab anybody ever, but it is clearly not possible.”
Dame Cressida said that the speed with which officers in Streatham responded to the attack was an “extraordinary achievement”.
She said armed officers who attended the incident were currently off the streets for “welfare” reasons.
An investigation involving 75 officers was ongoing, Dame Cressida said, but there was “no evidence” that the “horrible and really shocking” attack was “directed or enabled by anyone else”.
In response to the attack ministers want to introduce emergency legislation to make terror offenders serve more time in prison – but a former government adviser has warned those plans could lead to a legal battle.
Dame Cressida said “strong licence conditions” for people being released from prison must remain in place in the event of any changes to sentencing law.