The inquest into his death heard that police shot at the 28-year-old 20 times, in several bursts.
Khan had been chased onto London Bridge after murdering two victims in a knife rampage at Fishmongers’ Hall on 29 November 2019.
An inspector codenamed TC82, because of an anonymity order, told Monday’s hearing that Khan was believed to be reaching for what looked like a real suicide vest when the second barrage of shots were fired.
TC82, who was a Tactical Firearms Commander in the control room during the attack, said that while watching live footage of the incident he believed Khan could be holding a trigger.
“I feared the IED deonating,” he added. “Obviously because he’s on a bridge,that bridge could collapse and I could see members of the public walking – just going about their day-to-day business – underneath the bridge.”
TC82 recalled seeing Khan sit up minutes after he was initially shot and appearing to be “ready to detonate”.
He told the inquest he heard that the critical shot authorisation had been given at around the same time as multiple officers opened fire.
Matthew Middleditch, the technical lead of the Metropolitan Police’s explosives ordnance disposal unit, said Khan’s suicide vest “looked very realistic”.
He described it as “a very good facsimile of a person-borne IED”, which appeared to include separate compartments containing a white explosive substance and wires.
Mr Middleditch said that even as an expert, he “wouldn’t be able to recognise that as a hoax without having carried out specific actions on it”.
“If I was looking at Khan, I would have thought I was looking at a real person-borne IED,” he added.
Gary Wright, the explosives officer who inspected Khan’s vest on the day, said he believed he was dealing with a real bomb until removing the device and conducting technical checks.
The inquest heard that Khan had fashioned the device out of components including an Xbox 360 controller, USB charger and battery pack, and concealed it under a large coat before his attack.
The inquests into his victims’ deaths, which ended last month, found that Khan had targeted people linked to the Learning Together prison education scheme he attended while serving a terror sentence.
Jurors found that “omissions and failures” by the authorities charged with managing Khan after he was freed from jail in December 2018 contributed to the attack.
He was chased onto London Bridge by staff and attendees who had been at the rehabilitation event, wielding makeshift weapons including a fire extinguisher and narwhal tusk.
They pinned Khan to the ground before armed police arrived, and one officer shot khan twice after seeing his fake suicide vest.
The inquest heard he continued moving and sat up eight minutes after being shot.
Police fired a volley of 18 shots across a period of around 90 seconds. Khan stopped moving by 2.12pm, around 15 minutes after launching his attack, but was not formally pronounced dead until the scene had been declared safe an hour later. The inquest into his death continues.