The officer in charge of police who shot Jean Charles de Menezes says she was told five times by officers that he was 21 July bomb plotter Hussain Osman.
Brazilian Mr de Menezes, 27, was killed on a train at London's Stockwell Tube station after he was followed because he was wrongly identified as Osman.
Deputy Assistant Commissioner Cressida Dick told the Old Bailey why she asked armed officers to stop Mr de Menezes.
The Metropolitan Police denies breaking health and safety laws on 22 July 2005.
DAC Dick, who has since been promoted, was the commander in charge of overall operations on that day.
She told jurors on Thursday that a surveillance team had been deployed to Scotia Road, south London - an address linked to Osman - that morning.
Mr de Menezes was later followed by police, who believed him to be Osman, from a block of flats on Scotia Road to Stockwell Tube station, where he was shot seven times in the head on the train.
'Public safety'
DAC Dick told the court she had been told three times by a surveillance officer, codenamed Pat, and twice by her "silver" commander that the man officers had been following was Osman.
"The behaviours that were described - the nervousness, agitation, the sending of messages, the telephone, getting on and off the bus - added to the picture of someone potentially intent on causing an explosion"
Speaking on the first day of the Met's defence case, she said she believed Mr de Menezes had to be stopped from getting on the Underground "for public safety reasons".
Asked by the judge why she had acted without "100% positive identification", she replied: "I believed that they [officers at the scene] believed it was him but also that they could be wrong."
She said her handling of the situation had been proportionate.
DAC Dick added: "Firstly, I believe that the surveillance team believed it was him.
"Secondly, from the behaviours that had been described to me - given that I thought they thought it was him - it could, very, very well be him.
"The behaviours that were described - the nervousness, agitation, the sending of messages, the telephone, getting on and off the bus - added to the picture of someone potentially intent on causing an explosion."
'Very high risk'
DAC Dick said that, earlier that day, she had seen CCTV footage of another 21 July suspect - Ramzi Mohammed - going down the escalator at Stockwell tube station before he had tried to detonate a bomb near Oval station.
"That all added up - I cannot be certain - to someone who posed potentially a very high risk to the public," DAC Dick told court.
"The threat we were dealing with at that time was to the public transport system and to the Tube. We had two incidents - 7 July and 21 July."
Moments later, the SO19 specialist firearms team arrived at Stockwell Tube station.
"I then ordered 19 to do it," she told the court.
Parts of her handwritten log of events that day, written in the evening following the shooting, were read out to the court.
Her log recorded how Mr de Menezes had gone down the escalator into the Underground.
She wrote that he "must be detained and not allowed to travel on Tube for public safety reasons".
'No wires'
Another part of DAC Dick's log read: "Must be arrested by SO19. He is described as very jumpy and agitated. He has been on the phone and sending text messages."
It added: "If he were to enter [Stockwell station], I would have no contact with surveillance teams.
"However, he is not carrying anything, no wires visible. He is wearing denim, cannot rule out he is wearing secreted explosives or weapons."
DAC Dick wrote that she did "not think it right to allow such a subject to travel on the Tube".
Earlier on Thursday, DAC Dick denied claims the control room was "noisy and chaotic".
She was responding to prosecution claims that officers had to shout to make themselves heard in room 1600 - the 16th floor nerve centre at Scotland Yard.
The trial was adjourned until Friday.
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