A police marksman today recalled the moment he became convinced that Jean Charles de Menezes was a suicide bomber about to detonate his device.
The officer from Scotland Yard's elite CO19 specialist firearms unit, identified only as C12, said he believed he had to act to stop an atrocity.
He told an inquest how he came face to face with Mr de Menezes inside a cramped Tube train at Stockwell Tube station in south London on July 22 2005.
C12 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 scrum-like "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.
"He (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 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.
C12 described his feelings when he learned the next day that the man he shot was not Osman.
He told the inquest: "A sense of disbelief and of shock, sadness, confusion.
"Everything I have ever trained for - for threat assessment, seeing threats, perceiving threats and acting on threats - proved wrong.
"And I am responsible for the death of an innocent man. That is something I have got to live with for the rest of my life."
The inquest was adjourned until Monday, when C12 will be cross-examined by the Menezes family's barrister, Michael Mansfield QC.
Mr de Menezes' mother, Maria Otone, held back tears as she emerged from the hearing. Via her translator, Mrs de Menezes, who lives in Brazil, said hearing today's evidence had been "very difficult".
Flanked by her elder son, Giovani da Silva, she said: "It's been very difficult for us to listen to everything that's been said, but we want to be here every day because we want to hear the truth.
"I feel very strong and I have been brave and all this strength has been coming from God."