Video report by ITV News Security Editor Rohit Kachroo
The brother-in-law of one of the London Bridge attackers, who reported him to counter-terrorism authorities, says he wishes he had punched his relative during a row over his extremist views.
Usman Darr told ITV News Security Editor Rohit Kachroo he reported Khuram Butt to a counter-terror hotline after the argument, saying: "I did my bit...the system failed and eight people died."
Mr Darr was one of four family members who gave evidence during the inquests into the June 2017 attack, alongside Butt's widow Zahrah Rehman, brother Saad Butt, and sister Haleema Butt.
Summing up the evidence on Friday, coroner Mark Lucraft QC said he did not find the family members were "convincing witnesses" and that each had accepted "they should have done more at the time".
Recalling the argument he had with Butt, Mr Darr said: "We had a very detailed conversation about the murder of a Jordanian pilot who was captured in Raqqa. He was burnt alive.
"I could not understand the fact that my brother-in-law was justifying that murder."
Butt justified the death by comparing it to a battle which took place 1,400 years ago, where Muslims burnt trees because the enemies were using them for food and water - and therefore starved to death.
Butt claimed those responsible for the pilot's murder did so in revenge for the killing of people in a village hit by air strikes a day earlier.
In the bombing, children had been killed.
Recalling the argument, Usman said: "We were nose to nose, and I really wanted to hit him.
"I think it was just after he mentioned this particular ancient battle.
"Then mother had to come and intervene and separate us."
He continued: "I wish I would have hit him.
"Perhaps, maybe, his family would have taken more notice (of his extremist views)."
It was after this argument that Usman reported Butt to counter-terror officials for his views.
For the first time since the London Bridge attack, Usman Darr looks back at old text message conversations between him and Khuram Butt
"It was the September 20, 2015," he said.
"I searched on Google, rang the number, gave them the name, and told them that I had concerns about his extremist views."
Usman blames the authorities for not properly following up on the intelligence.
He told ITV News: "They missed something. They had to miss something.
"How can three boys, in this day and age, get instructed to carry out such an attack and the intelligence doesn't know about it?"
"We always talked about how the Muslim community does not do enough to curb extremism in our community.
"I did my bit.
"The system failed and eight people died."
Speaking about the attack, he said his brother-in-law and the two other attackers are responsible for "indiscriminate killing".
"He doesn't know that the woman he killed could have been a nurse that looked after his grandmother.
"The man he ran over could have been a doctor that helped his mother.
"It's just senseless.
"I just don't understand the psyche behind this."