London Mayor Sadiq Khan on Saturday said "big questions" needed to be answered following the revelation that the London Bridge attack suspect had served jail time for terrorism offences.
He also repeated that London would never give in to extremist attackers and would be open for business as usual over the Christmas holidays.
The mayor was speaking as police searched homes in Stafford, 225 kilometres northwest of London, in connection with the investigation.
A man and a woman were killed and three other people were wounded when the suspect, named as 28-year-old Usman Khan, stabbed them, before he was tackled by bystanders and shot dead on Friday by three officers who surrounded him and fired twice.
Khan, a British man whose family is from Pakistan-controlled Kashmir, was convicted for terrorism offences in 2012 for his part in an al-Qaeda-inspired plot to place a bomb in a toilet at the London Stock Exchange. He was released in December 2018, subject to conditions, before the end of his sentence.
Wearing a fake suicide vest and wielding knives, Khan went on the rampage shortly before 2 p.m. on Friday before being wrestled to the ground and then shot by specialist armed police. He was wearing an electronic tag and had reportedly been invited to attend a conference on criminal rehabilitation on the northern side of London Bridge.
Watch: Man with knife wrestled to the ground (Warning: Graphic content)
It's believed the attack started at the Learning Together conference, organized by Cambridge University at Fishmongers Hall, before spilling outside onto the bridge. Members of the public intervened, with several captured on camera pinning Khan to the ground.
"I don't think that it's right that somebody is convicted of a serious offence like what this man is convicted of, should be automatically released," the London mayor said.
He said he believes in prisoner rehabilitation, but also in the need for those released to be "properly supervised with a system that is effective."
Khan joined the capital's police chiefs on a tour of the London Bridge area on Saturday. He earlier told spoke to London-based radio station LBC about the IPP (imprisonment for public protection) model of justice for some prisoners, which was abolished in December of 2012.
Khan said the "indeterminate sentences for the protection of the public" meant that if "found guilty of a very serious terrorist offence, you will not be released until you can persuade the authorities that you are no longer a danger to the public."
He added judges have to give the "right sentences" because people convicted of very serious offences can still be a danger to the public and can be released after serving just half their sentences.
Khan was imprisoned after receiving a sentence of IPP, but he and two others successfully appealed those sentences. In 2013, the court of appeal revised Khan's sentence to 16 years and told him he would have to serve a minimum of eight years.
Prime Minister Boris Johnson also spoke Saturday about the sentencing of people convicted of terrorism offences, saying they should not be allowed out of prison early.
"I think that the practice of automatic, early release where you cut a sentence in half and let really serious, violent offenders out early simply isn't working, and you've some very good evidence of how that isn't working, I am afraid, with this case," Johnson said.
The U.K. parole board issued a statement on Saturday saying it appears Khan was released automatically "on licence" — which means he had to meet certain conditions or face recall to prison — without ever being referred to the board.