The suspect in front of me had a knife pressed to the throat of a woman hostage and was shielding himself with her body. As I raised my police issue Glock pistol he screamed at me to get away.

Suddenly the weapon felt very heavy and my heart began thundering in my chest. The woman was screaming. I was vaguely aware of others taking cover. Everything seemed to happen in slow motion, but still too quickly for me to make sense of what was happening.

I was just seconds into my session at the Metropolitan Police Specialist Training Centre in Kent, where Scotland Yard's CO19 specialist firearms unit trains — and already it seemed that I had lost control of the situation.

“Armed police, drop the weapon!” I shouted at the large interactive video screen in front of me. But the suspect just turned his back.

I thought about taking a shot, but we had just been briefed about how some bullets would go through a person for some distance and in my panicked state I could not remember which bullets I was meant to have in my weapon.

Would the bullet go through him and hit the woman? Before I could figure this out he had turned to face me. Should I try a head shot and risk hitting the hostage or wait for a better opportunity? Or would that be too late?

As I raised the gun to aim at his head he pushed the woman away from him. I fired at his torso just as he sat down, his knife arm dropping limply at his side.

Was he still a threat? If I had waited a heartbeat longer would I have had to shoot?

Fortunately for me, it didn't matter what the answer was. This was, after all, just an exercise. But for the officers who shot Jean Charles de Menezes the predicament they faced that day — and its consequences — was all too real.

The very different recollections of police and witnesses of the circumstances surrounding the shooting were at the core of the inquest. One thing that my experience on the training course did reveal was how decisions taken in the heat of the moment can look very different in the cold light of the following day.

At the end of the exercise I relaxed and lowered my gun. Too soon it appeared. Everyone else in the room saw the man jump up, take two steps to the woman and stab her before running off. Worryingly, I did not remember seeing that at all.

As soon as the screen went blank Inspector Tony Kalli, a veteran firearms officer, began firing questions: “What was the woman wearing? Did he really have a knife? What was he shouting?” I could not remember. I could not remember what the woman was wearing or what the gunman was shouting, but I was sure he was carrying a knife.

Mr Kalli, who has been involved in many armed operations, explained: “The body starts to shut down and close down the outer senses and concentrate on the senses that will save your life.” Hearing is one of the first to go.

He said that although an officer would have taken in all the information during a shooting it could be an hour or a day before he was able to remember it all. That is why they do not have to give an immediate report into what has happened.

Later, Mr Kalli gave me an unloaded pistol and told me to point it at him and fire when I believed I was in danger. He then picked up another unloaded gun, by its barrel.

“Am I a threat now...?”

BANG. In a split second, while he was talking, he had transferred the gun to his right hand — and fired at me.

Times Recommends

• Charles blazes trail as sun king
• Thalidomiders win payout 50 years on
• Libel gag on talk of 'medical hurricane'