The armed police officer who gunned down Jean Charles de Menzes has broken down in tears as he described the shooting.
The officer from Scotland Yard's elite CO19 specialist firearms unit, identified only as C12, told the Menezes inquest he was convinced the the 27-year-old Brazilian electrician was a suicide bomber about to detonate his device.
He said he believed he had to act to prevent an atrocity at Stockwell Tube station in south London.
C12 described how he came face to face with Mr de Menezes inside a cramped Tube carriage on July 22 2005.
He said he recalled that the innocent Brazilian got up and walked towards him - and kept moving even after he shouted "armed police" and pointed his gun at him.
He said: "He continued on his forward momentum towards me. It was at that stage that I just formed the opinion that he's going to detonate, he's going to kill us and I have to act now in order to stop this from happening."
Mr de Menezes was shot seven times in the head at point-blank range after being mistaken for failed suicide bomber Hussain Osman.
C12 choked back tears as he described how he shot Mr de Menezes three times because he believed he was "going to detonate, going to kill us".
He recalled a "ruck" as he pointed a gun towards Mr de Menezes's head while surveillance officer Ivor pinned the Brazilian down.
He said: "I brought the handgun up from my leg and pointed it up to shoulder level. Ivor immediately grappled him into his seat.
"It just reconfirmed to me that I had to use force as soon as possible. If there was any alternative, you must believe me, I would have taken it but I did not have any alternative.
"I did not believe I had an alternative and if I did not act, members of the public would be killed, my colleagues would be killed and I would be killed. I had a duty to protect the public."
He said his Glock pistol was next to Ivor's head as he opened fire.
He said: "We were all together and it was a bit of a ruck. As Ivor got hold of him, I am virtually on the top of Ivor bringing my weapon up.
"I remember the gun coming into contact with Ivor's head in order to get to Mr de Menezes.
"I do not know how close it was. I honestly believed this man to be a suicide bomber."
C12 was asked why he shot Mr de Menezes three times.
He replied: "I had to be certain that life was extinct, that there wasn't any more threat, that this person couldn't detonate a bomb.
"Our training is, or has been, that a critical shot is a single shot, probably to the base of the skull, which will cause instant paralysis.
"I was not in a position to get there to do that. I fired the number of shots because I detected moments, albeit it might have been movement caused by the bullets."
C12 also said he would not have fired at Mr de Menezes if he had remained seated or if he had stopped approaching him when he shouted "armed police".
The inquest was halted briefly after C12 became distressed by his recollections and could not continue. When it resumed, he recalled being "covered in blood" after the shooting.
He and C2, the other firearms officer who shot Mr de Menezes, checked each other to ensure they had not been wounded in the incident.