Usman Khan, who killed two people in the London Bridge terror attack last week, was a convicted terrorist who had been released halfway through his 16-year sentence.
Prisoners can typically expect to serve around half of their sentence in prison before being released on licence.
They must adhere to certain conditions, which can involve curfews monitored by an electronic tag and meetings with a probation officer and can be recalled to prison at any time.
UK prime minister Boris Johnson said on Saturday there were probably about 74 people with terror convictions who had been released on licence.
Their conditions are currently under review.
Khan was convicted in January 2012 for engaging in conduct in preparation for acts of terrorism.
He had been part of a plot inspired by al-Qaeda to set up a terror camp in Pakistan and bomb the London Stock Exchange.
Khan was released in December 2018 - less than seven years into a 16-year prison sentence - and was wearing an electronic monitoring tag.
Britain's counter-terror police chief Neil Basu said on Saturday that Khan had complied with an "extensive list of licence conditions" following his early release.
How was Khan released?
In 2008, the Labour government changed the rules on extended sentences.
Offenders were automatically released halfway through their sentences rather than being reviewed by a parole board.
The Conservative-Liberal coalition government changed the rules again in December 2012 so that those serving more than 10 years could be freed only after two-thirds of their sentence and only with the approval of a parole board.
However, this did not apply to Khan as he was convicted under the previous laws and the new ones were not applied retrospectively.
Johnson is calling for an end to automatic early releases as well as the complete end of release on licence for people convicted of terror offences.
"If you are convicted of a serious terrorist offence, there should be a mandatory minimum sentence of 14 years - and some should never be released," he said.
Johnson also said terror and extremism convicts should serve the full period specified by the judge.
Jack Merritt was one of the two people murdered by Khan on Friday.
The 25-year-old was a course coordinator of the Cambridge University Learning Together prison rehabilitation programme.
His father David Merritt said his son "would not wish his death to be used as the pretext for more draconian sentences or for detaining people unnecessarily".