The police officers who shot Jean Charles de Menezes dead were "convinced" at the time that he was a suicide bomber, an inquest has heard.
Coroner Sir Michael Wright said the two officers had thought Mr de Menezes was about to detonate a device on the Tube.
His comments came at the opening of a three-month inquest in London.
The Brazilian, 27, was shot at Stockwell Tube station, south London, in mistake for a bomber on the day after the failed 21 July 2005 attacks.
Taking the inquest jury through events leading up to the electrician's death, Sir Michael listed a number of occasions where officers were unclear whether or not they thought they were pursuing a bomber.
He told them of differences between what was being relayed on radio and logged in the Scotland Yard control room - and how officers were interpreting the information.
Mr de Menezes was killed instantly. He could hardly have had an opportunity to appreciate was happening
Coroner Sir Michael Wright
Sir Michael said that as Mr de Menezes entered the Tube at Stockwell, no member of the surveillance team had positively identified him as Hussain Osman, one of the four 21/7 bombers they were hunting.
Turning to the decision of the two marksmen to shoot, Sir Michael said they had jointly fired nine rounds, seven of which entered the Brazilian's head at point blank range.
"Both officers state that they were convinced that Mr de Menezes was a suicide bomber, that he was about to detonate a bomb, and that unless they prevented him from doing so, everyone in that carriage was going to die," said Sir Michael.
"Each officer says that he was convinced that an instant killing was the only option open to him.
"Mr de Menezes was killed instantly. He could hardly have had an opportunity to appreciate was happening."
'No terrorism link'
The inquest will continue with the jury visiting key locations on Tuesday including Stockwell Tube station and flats in Scotia Road in Tulse Hill, south London, where Mr de Menezes lived.
The jury will consider whether or not Mr de Menezes was unlawfully killed.
Former High Court judge Sir Michael, assistant deputy coroner for Inner South London, had earlier sworn in the jury of six women and five men.
He told jurors: "The facts of the case are that two firearms officers shot dead Mr de Menezes because they thought he was a suicide bomber, but the facts were that Mr de Menezes was in no way associated with any form of terrorism."
Sir Michael added: "It will be for you to consider what level of identification was made at different stages, what was communicated to the firearms officers, and what those officers believed the position to be."
He said the inquest was a fact-finding exercise and "not a forum to determine culpability or compensation, still less to dispense punishment".
'Pain and injustice'
Three of Mr de Menezes' cousins, Alex and Alessandro Pereira and Patricia da Silva Armani, and members of the Justice 4 Jean campaign were at the inquest.
They and other relatives have campaigned for police officers involved in the shooting to be prosecuted.
Patricia da Silva Armani reads out a statement from the family
Outside the court, Ms da Silva Armani said: "Today is the first day of a process, which we hope will bring my family closer to the truth.
"We are hoping that, at the end of the inquest, we will get the answer we need about how my cousin died.
"Hearing the coroner tell the history again bought back the pain and injustice of Jean's killing. This will be a long and painful three months for us, but we will be here until the end - to get to the truth and to get justice for Jean."
Mr de Menezes was shot dead on 22 July 2005 by specially trained Metropolitan Police firearms officers.
High public interest
Teams of undercover officers had trailed him across south London after he left flats which were under surveillance.
The inquest will hear from 75 witnesses, including 48 serving police officers who have been granted anonymity, and Tube passengers.
The first police officer will appear later in the week.
Among those who will be speaking for the first time will be policemen codenamed C2 and C12, the two specialist firearms officers who shot Mr de Menezes dead.
The inquest is being held at the John Major conference room at the Oval Cricket Ground because of the scale of the proceedings and level of public interest.
There have been five inquiries relating to the death and its aftermath, including a criminal trial.
In 2007, an Old Bailey jury found the Metropolitan Police guilty of breaching health and safety laws, after hearing about the events leading up to Mr de Menezes being shot.
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