It was a snippet of conversation, along with other intelligence about a plot to bomb the London Stock Exchange, that prompted British police to arrest Khan - then 19 years old - and a group of older men on December 20, 2010.
"This individual was known to authorities", said Britain's top counter terrorism officer, Assistant Police Commissioner Neil Basu. He killed Jack Merritt, 25, a course leader of the program and a woman who has not yet been named.
Mr Merritt was described by his father on Twitter as a "beautiful spirit".
Health officials said two of the wounded were stable and the third had less serious injuries.
But Brexit has raised questions about the extent of continued cooperation with Europe on security and intelligence matters.
"[Khan] had been released a year ago on the provision that he had agreed to wear an electronic tag, submit to curfews and restrictions to his movements", Lucy Fisher, defense editor of The Times in London, said in an interview on NPR's Weekend Edition Saturday morning.More news: Apple’s Newest iPad Is Already Cheaper Than It Was on Black Friday
Authorities said Kahn was also found guilty of making longer-term plans, including starting a "terrorist military training facility" in Pakistan. He said extra police patrols on the streets would be added "for reassurance purposes".
The British government on Saturday vowed a "full review" as the justice system came under intense public scrutiny following the announcement that Khan was a convicted terrorist.
"Usman Khan was a loyal and integral member of Choudary's inner-circle and we know him to have been highly regarded by Choudary", the Henry Jackson Society's report said.He had been secretly taped plotting attacks and talking about martyrdom as a possibility. The preacher was released from prison in 2018 but is under heavy surveillance and a curfew.
Following the deadly attack on London Bridge that took place on 29 November, police said that the perpetrator was not unknown to law enforcement and was on parole after serving a sentence on terrorism-related charges.
Police on Saturday were searching an apartment block in Stafford, 150 miles (240 kilometres) northwest of London, for clues.
Sources revealed that Khan was attending a rehab meeting following his early release from prison. The main political parties suspended campaigning in London as a mark of respect.More news: Pick up a Nintendo 2DS XL Argos' Black Friday Sale for £95
"I am devastated to learn that today's hateful attack on London Bridge may have been targeted at staff, students and alumni", University of Cambridge Vice-Chancellor Stephen Toope said in a statement posted online.
Al Jazeera's Rory Challands, reporting from London, said Merritt was a coordinator for prisoner rehabilitation programme Learning Together and had been attending the conference on Friday.
Khan was shot and killed by police after flashing what appeared to be a suicide vest.
Eyewitnesses' account claimed that a group of people tried to tackle the suspect on the ground before the cops arrived. Gunshots followed. Another depicted a man in a suit holding a long knife that apparently had been taken from the attacker.
"The person who carried out the London attack. was a fighter from the Islamic State, and did so in response to calls to target citizens of coalition countries", IS said, referring to a multi-country alliance against the group.
She also paid tribute to those who risked life and limb to prevent more casualties.More news: Charles Leclerc says he and Sebastian Vettel share responsibility for Brazil crash
"What's remarkable about the images we've seen is the breath-taking heroism of members of the public who literally ran towards danger not knowing what confronted them", Khan told reporters, adding that man had been wearing a fake bomb.