A Dutch blogger who reported the Streatham terror attacker to UK authorities has told Sky News he was one of "the most violent" terrorists he had come across online.
Azazel van den Berg, who runs an anti-extremism blog, reported Sudesh Amman to the police before the attacker's arrest in 2018.
He was monitoring a pro-IS group on the messaging service Telegram in April 2018 when he came across Amman using the fake name Abu Malik to speak to other jihadists online.
Screenshots from Mr van den Berg's blog show the Streatham attacker, who was shot dead after stabbing two people in Streatham last Sunday, joking with other jihadists about playing Call of Duty "in real life" and making "a kuffar drown in his own blood". Kuffar is an Arabic term meaning non-believer.
Mr van den Berg told Sky News: "[Amman] was more aggressive [than other members of the group].
"He was fearless.
"Another guy said to him: 'Look out for the police - they see what you say in here', but that wasn't a concern for him.
"He said: 'The police already know me'."
The decision to alert the authorities came after Amman sent a string of messages calling for UK extremists to target a Malaysian LGBT rights activist at Speakers' Corner in Hyde Park on the following Sunday.
Amman had shared a photograph of a knife along with two firearms against a black Shahadar flag with the caption "Armed n rdy #april" just two days before encouraging an attack in Hyde Park.
It was Mr van den Berg's tip off that prompted UK authorities to arrest Sudesh Amman for preparing acts of terrorism in May 2018.
He was sentenced to three years and four months in prison after he pleaded guilty to six charges of possessing documents containing terrorist information and seven of disseminating terrorist publications in November of that year.
But Amman was released automatically on license after serving less than half of his sentence following measures implemented in 2008.
He attacked two people on Streatham High Street just ten days after his release.
Mr van den Berg, who has monitored online extremism for years, told Sky News: "He was so violent, I was shocked that they let him free. That guy was crazy, really religiously crazy."