Video report by ITV News Correspondent Angus Walker
The 28-year-old was released on licence last year after serving half of his 16-year sentence but was able to embark on a deadly stabbing spree in central London on Friday, killing a man and a woman and injuring three others.
But during his prison sentence, he wrote a letter from his HMP Belmarsh prison cell, claiming he was “immature” when he committed the offence and now wanted to “learn Islam and its teachings” through a “deradicalidation course”.
In the letter obtained by ITV News, Khan wrote: “I would like to do such a course so I can prove to the authorities, my family and soicity (sic) in general that I don’t carry the views I had before my arrest and also I can prove that at the time I was immature, and now I am much more mature and want to live my life as a good Muslim and also a good citizen of Britain.”
The attacker was attending a prisoner rehabilitation event organised by the University of Cambridge, where he "threatened to blow up" Fishmongers' Hall, where the conference was taking place, shortly before 2pm.
Vajahat Sharif, the lawyer who represented Usman Khan says it's "disturbing" that he carried out this Islamic State style attack as he'd spoken out against their ideologies.
He told ITV News: "He initially presented himself as a confused young man. Over the course of time he was quite a pleasant individual to deal with.
"He was critical of violent extremism. He understood it was wrong so this is quite astonishing."
ITV News understands that Khan was invited to do a “risk assessment and formulation” in 2012, while in Category A HMP Belmarsh - the first step of a deradicalisation programme.
However, it is understood that Khan repeatedly refused the risk assessment test and formulation, a process which would have been taking place before he wrote the letter.
Eventually, in 2014, he seemed to change his mind and went ahead with the risk assessment and formulation and was then on a programme most likely continuously until the end of the custodial part of sentence.
It is understood that once released from prison he went on to a Desistance and Disengagement Programme (DDP) which focuses on individuals who are subject to court-approved conditions, including all terrorism and terrorism-related offenders on probation licence).
According to the Ministry of Justice, a prisoner serving a sentence such as Khan's would have been required to take part in a deradicalisation course.
The Home Office has been contacted for comment.