He had also once written a list of "goals in life" which included the desire to die as a martyr.
Police have named Sudesh Amman, 20, as the attacker. He was killed on a street in the south London district of Streatham on Sunday afternoon by armed officers who had been carrying out surveillance on him.
In November 2018, Amman had pleaded guilty to 13 offenses of possession of terrorist documents and disseminating terrorist publications and the following month he was sentenced to more than three years in prison. He was released last month having served half the sentence.
Amman was 17 and living at home in Harrow, north London, with his mother and four younger siblings when he first began committing terrorism offenses, according to authorities. Police became aware of his activities in April 2018, and he was arrested in a north London street a month later.
When officers examined his computers and phone, they found he had downloaded material about making explosives and carrying out terrorist attacks, court documents show.
Amongst the material found were documents titled "How to make a bomb in the kitchen of your mom" and "U.S. Army Knife Fighting Manual techniques" and "Bloody Brazilian Knife Fightin’ Techniques."
Messages showed that he had discussed with his family, friends and girlfriend his extreme views and desire to carry out an attack, often focused on using a knife.
"In online chat you tell your girlfriend that you have declared a pledge to Islamic State and that you wish to purge society and carry out acid attacks," Judge Mark Lucraft said when he sentenced him to prison.
"You speak about preferring a knife attack to the use of bombs and ask about having a knife delivered to her address."
In December 2017 Amman posted a picture of Islamic State leader Abu Bakr al-Baghdadi, who was killed during a U.S. raid in Syria in October.
He also described Yazidi women as slaves and said the Koran made it permissible to rape them. In another message he encouraged his girlfriend to behead her parents.
Police said he had shared an online al-Qaeda magazine with his family and while in a discussion about school with a sibling he wrote he would "rather blow myself up."
Amman's mother told Sky News her son was a "nice, polite boy" who had been radacalised by watching extremists Islamist material online and became more religious while in prison.
She said she had spoken to him just hours before the attack to say he wanted some of her mutton biryani and when she last saw him days earlier he had appeared normal."He was fine when I went to see him," she told Sky. "He became more religious inside prison, that's where I think he became radicalized. He was watching and listening to things online which brainwashed him."