At 1.55pm on Friday, Usman Khan descended the grand staircase leading to the ornate banqueting suite in the basement of Fishmongers' Hall near London Bridge.
There, academics and former prisoners had gathered for a conference on rehabilitation.
Khan, a convict released on licence and wearing an electronic tag, had been taking part in the conference.
No one gave him a second look when he slipped away. Minutes later he was back – this time with knives taped to his hands and wearing a fake suicide belt – and moving purposefully across the room.
In the words of one academic there, Khan 'suddenly just flipped'.
A killer at bay: Rampaging knifeman Usman Khan, left, is halted on London Bridge by a Polish chef named Lukasz, centre, armed with a narwhal tusk, and an unidentified man wielding a fire extinguisher
That, at least, is how it appeared. In reality, what came next – a murderous rampage lasting just eight minutes that would leave three dead, including Khan himself – seems to have been well planned.
Khan, 28, had been jailed in 2012 for his part in an Al Qaeda-inspired terror group's plot to bomb the London Stock Exchange.
He had been invited to the conference, Learning Together, run by Cambridge University's Institute of Criminology, as a rehabilitated prisoner who had turned his back on his fanatical past.
But as the 150 or so delegates would discover, nothing could be further from the truth.
Making a break: Khan, above centre, tries to escape the have-a-go heroes but more brave passers-by join in
One of the conference organisers, Jack Merritt, 25, was the first to fall victim to Khan's onslaught.
An unnamed woman, believed to be just inside the entrance to the hall, was fatally injured as the fanatic slashed his way out of the building.
Amid chaos and panic, a witness recalled seeing 'two, three people stabbed' with 'one minor injury and the other two were proper bleeding'.
Anyone crossing his path was likely to receive the same merciless treatment. But Khan had reckoned without the bravery of some of those at the conference.
One, a Polish chef named Lukasz, is believed to have grabbed a 5ft narwhal whale tusk that hung over a doorway in the hall, home of the Worshipful Company of Fishmongers, and began running after Khan as he fled towards London Bridge.
The killer cornered: Police and the London Bridge heroes pin down the attacker
Another man picked up a fire extinguisher and joined the chase.
According to a source, all those involved in tackling Khan – with the exception of Mr Lukasz – were ex-offenders, either on day release or on licence.
One of them was James Ford, convicted of killing Amanda Champion, 21, who had a mental age of a 15-year-old, in Ashford, Kent, in 2014.sonos sonos One (Gen 2) - Voice Controlled Smart