Jack Merritt, 25, of Cottenham, Cambridgeshire, and Saskia Jones, 23, of Stratford-upon-Avon, Warwickshire, were killed when 28-year-old Usman Khan went on a stabbing spree before being shot dead by police.
After his 2012 conviction in a plot to blow up the London Stock Exchange and other sites, Khan was initially sentenced to 16 years in prison, but was released early "on license", meaning he had to meet certain conditions or risk re-imprisonment.
Khan, a British national from Stoke in central England, was handed an indeterminate sentence for public protection in 2012, with at least eight years in prison.
Moments later armed police officers arrived on the scene and shot him dead.
Merritt was a course coordinator for the programme, which seeks to "bring together people in criminal justice and higher education institutions to study alongside each other in inclusive and transformative learning communities".
But Tory David TC Davies said it was right to review the early release of those convicted of terrorism offences.
"It's not lenient policies that are to blame, it's the destruction of the probation service that is supposed to monitor and supervise prisoners after release, & rehabilitation services", he added.
Former Labour Welsh Secretary Lord Hain said it was important to support the police and not give "knee-jerk responses".
"We're supporting the police through this very hard investigation".
Liberal Democrat Ed Davey said on Sky News that the prime minister was making "political capital out of a tragedy" and misleading people about what the law says.
The man with the fire extinguisher has been named as a convicted murderer who was jailed in 2004 for the murder of a young woman, and is reported to be in the final days of a rehabilitation program which included him attending an event at the Fishmongers' Hall.
A large police investigation remained underway late Saturday at a property in Staffordshire in the English Midlands where it's believed Khan was living before the attack.
A British Transport Police officer in a suit and tie who also intervened was seen carrying a large knife away. He was released in December, 2018, (his time served took into account the period he spent in jail awaiting trial) and placed on a 13-year probation program that included wearing a Global Positioning System ankle bracelet and regular monitoring by police.
Inmates are usually released half-way through this type of determinate sentence - and time spent in custody before trial may have been taken into account.
The Parole Board said it had no involvement in his release and that it appeared to have happened automatically as required by law.
London police said they were treating the attack as an act of terror.
"It is a mistake to allow serious and violent criminals to come out of prison early and it is very important that we get out of that habit and that we enforce the appropriate sentences for unsafe criminals, especially for terrorists", the PM said on Friday ahead of an emergency meeting with the government's Cobra committee.