Usman Khan: The 28-year-old who launched London Bridge attack

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 Dec. 20, 2010.

Sentenced to a minimum of eight years in prison in 2012 with a requirement that the parole board assess his danger to the public before release, he was set free in December 2018 – without a parole board assessment.

On Friday, he strapped on a fake suicide vest, armed himself with large kitchen knives and went on the rampage at a conference on prisoner rehabilitation beside London Bridge.

Confronted by bystanders, including a Polish man brandishing a narwhal tusk he had grabbed from the wall of Fishmongers’ Hall, Khan was wrestled to the ground. Three armed police officers surrounded him. They fired twice. He was dead.

“This individual was known to authorities,” said Britain’s top counter terrorism officer, Assistant Police Commissioner Neil Basu. “A key line of enquiry now is to establish how he came to carry out this attack.”

It is so far unclear why Khan, now 28, began his rampage on London Bridge – the scene of another deadly attack before the 2017 election. Then, three militants drove a van into pedestrians before stabbing people in the surrounding area, killing eight and injuring at least 48.

Khan was part of a group of militants from the English city of Stoke which forged close links with militants from London and the Welsh capital Cardiff.

The London and Welsh parts of the conspiracy had ambitions to place a bomb in a toilet at the London Stock Exchange.

“The Stoke group was, and was considered to be, pre-eminent,” British judge Alan Wilkie said when he sentenced Khan in 2012. “They regarded themselves as more serious militant than the others.”

When sentencing Khan in 2012, Wilkie said that he was so dangerous that he was imposing a so-called imprisonment for public protection (IPP) indeterminate sentence of eight years.

After Khan appealed his sentence, appeal court judges in 2013 quashed the indeterminate period of incarceration and he was given a determinate sentence of 16 years – meaning he could be released after serving half of his term.

Court of Appeal judges said at the time that the Parole Board should consider whether those convicted were safe enough to be released. But the Parole Board said on Saturday that it had not been involved in deciding Khan’s release.

“The Parole Board can confirm it had no involvement with the release of the individual identified as the attacker,” the board said, adding that Khan “appears to have been released automatically on licence (as required by law), without ever being referred to the Board.”

Prime Minister Boris Johnson, seeking re-election on Dec. 12, said it was important to enforce terrorism-related sentences.

“It does not make sense for us, as a society, to be putting people convicted of terrorist offences, serious violent offences, out on early release,” he said on Saturday after visiting the scene of the attack.