Lindt Cafe manager Tori Johnson was ordered to kneel on the floor by siege gunman Man Haron Monis, who then shot him in the back of the head with his sawn-off shotgun from just 75cm.

It was a horrific end to the siege that had begun after Iranian refugee Monis walked into the Martin Place cafe at 8.33am the day before, ate a slice of chocolate cake, drank a cup of tea and then asked to speak to the manager.

Staff could tell from Mr Johnson’s body language that what he was being told was bad.

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Mr Johnson called over one of them and said: “I need you to get my keys from the office and lock the doors. We are closed.. Everything is OK, tell the staff to be calm.”

Monis then stood up, pulled on a vest and a bandanna, got the pump action shotgun out of his blue bag and said: “This is an attack, I have a bomb.”

He ordered Mr Johnson to make the chilling Triple O call: “Australia is under attack by Islamic State and there are a number of radio controlled bombs situated in Martin Place, Circular Quay and George Street.”

The bomb threats were a hoax but the shocking details of the 17 hours inside the Martin Place siege were revealed yesterday as state coroner Michael Barnes began what will be an exhaustive and emotional inquest into what led up to the siege and whether the deaths of Mr Johnson, 34, fellow hostage Katrina Dawson, 38, and Monis, 50, could have been prevented.

“Rarely have such horrifying events unfolded so publicly,” Mr Barnes said yesterday as he opened the inquest at Glebe Coroner’s Court.

“Overlaying the intense personal suffering on display were fearsome themes calling up wider, more far-reaching threats that understandably terrified many, even among those who only saw it from afar.

“Justifiably then,these events have precipitated an outpouring of emotion: anguish, anger resentment and even despair.”

With a “crack” team of more than 30 detectives and experts led by homicide squad detective inspector Angelo Memmolo, Mr Barnes said they would do everything in their power to find out why the siege happened and whether it could have been prevented or responded to more effectively by police.

As the first hostage to escape the siege, John O’Brien, 83, and some family members of Mr Johnson’s family listened in the courtroom, counsel assisting Jeremy Gormly SC said the hostages were the eyes, ears and memory of what happened inside the cafe.

He said the tapes of any highly-paid interviews they have done with the media would form part of their evidence. All but one had made police statements before they did their TV and magazine deals.

Mr Gormly said a 3D reconstruction has been created of the scene as Tactical Operations Unit officers entered the cafe and ended the siege seconds after Mr Johnson was shot at 2.14am on December 16.

Every shot would be accounted for, including the five fired by Monis. Segments of the windows and walls in the cafe struck by bullets had been removed for examination.

Monis was reloading his shotgun when he was killed by at least two police bullets to the head and another 11 other police bullets or fragments smashing into his body.

Mr Gormly said that barrister Ms Dawson, a mother-of-three, died after six fragments of police bullets ricocheted into her, one of them fatally severing a major blood vessel.

Monis’ background and mental issues will play a crucial part of the inquiry and forensic psychiatrist Dr Jonathan Phillips will provide a psychiatric profile of the self-styled shiek who was on bail for murder and sexual assault charges at the time.

Outside court, Mr O’Brien said he had felt emotional during the hearing and it had been upsetting for Mr Johnson’s family.

The inquest was adjourned to a date to be fixed. It will be held in segments as evidence is finalised.