In a statement overnight, the U.K.'s Met Police said the individual was released from prison in December 2018 on license, adding that a key line of the enquiry was now to establish how he came to carry out this attack.
The attacker had attended a prisoner rehabilitation event called "Learning Together" on Friday afternoon at Fishmonger's Hall on the north side of the bridge. The Met believes the knife attack began inside before he left the building and proceeded onto the bridge, where he was detained and subsequently shot dead by armed officers.
The suspect was initially restrained by members of the public and he appeared to be wearing a bomb vest which was later said to be "a hoax explosive device."
One man and one woman were killed during the attack. Three others, a man and two women, were also injured and remain in hospital. Health officials have said one of the injured is in a critical but stable condition.
The police said it was not actively seeking anyone else in relation to the attack, but were carrying out searches at an address in Staffordshire, in the West Midlands of England, where Khan is believed to have lived.
"Public safety is our top priority and we are enhancing police patrols in the City and across London," Assistant Commissioner Neil Basu for the Met Police said in a statement. Earlier this month, the country downgraded its terrorism threat level from "severe" to "substantial."
The BBC reported that Usman Khan was sentenced to "indeterminate detention" in 2012 with a minimum jail term of eight years, adding that it would have allowed him to be kept in prison beyond that minimum term. The prosecution at the time said the plotters, including Khan, had discussed attacking the London Stock Exchange as well as pubs in the English city of Stoke.
In 2013, the U.K.'s Court of Appeal quashed that sentence and replaced it with a 16-year-fixed term with half of it being spent in jail.
The U.K.'s Times Newspaper reported on Saturday that the convicted terrorist was released from jail last year after agreeing to wear an electronic tag to monitor his movements.
Friday's incident came 2½ years after eight people were killed and 48 were injured in a terrorist vehicle-ramming and stabbings along London Bridge, which links the capital's business district with the south bank of the River Thames.