The firefighter in charge during the first hour of the Grenfell Tower inferno broke down in tears as footage of the burning block was shown to an inquiry.
Michael Dowden, watch manager from North Kensington fire station, was the incident commander at the June 14 2017 fire until just before 2am.
His initial response to the fire was examined in forensic detail during a second day of evidence at the inquiry into the tragedy.
The 14-year veteran of the London Fire Brigade was asked to watch mobile phone footage of the approximate time he recalled noticing Grenfell Tower's cladding was alight - 1.19am.
When images of the burning tower were played on the monitors, the probe's top lawyer, Richard Millett QC, sensed his distress and halted proceedings.
He was offered a five-minute break by chairman Sir Martin Moore-Bick.
Asked if he would like longer, the officer said: "10, please."
He then used a handkerchief to wipe tears away from his eyes and was led from the room by an usher.
Mr Dowden began showing signs of strain at Holborn Bars on Monday, prompting Sir Martin to warn he was "increasingly concerned" about his wellbeing.
As his account of events was subjected to detailed scrutiny on Tuesday, the officer stressed that the "sensory overload" of that night had impacted his memory.
He told the inquiry he had begun feeling "uncomfortable" about the fourth-floor flat fire shortly after 1.16am, roughly 22 minutes after the first 999 call.
He said: "I knew there was a (breathing apparatus) team inside the compartment and I couldn't understand why that wasn't being suppressed."
Mr Millett referred to footage from 1.12am - showing embers falling from the kitchen window - and asked why he didn't feel uncomfortable at that stage.
The officer replied: "The decision and choices... they had to be very, very quick - I didn't have the benefit to work from a timeline."
It was heard that crew manager Christopher Secrett had radioed from the fourth-floor flat several minutes later saying the blaze was out.
Mr Dowden said: "From what I could see on the outside that wasn't happening."
The fire was "sparking and spitting" at this stage, the inquiry was told.
Asked what he thought the falling material was, he said: "At that point it was something on the external of the building."
"I didn't know at that point what I know now, in terms of flammable cladding, the ACM cladding.
"If we were aware of that risk and that hazard at that point as we are now as an organisation we would have put things in place, but I wasn't aware of this cladding material put on to the external envelope of the building."
Combustible cladding installed on the building's face during a botched refurbishment has been blamed for the rapid spread of the fire.
Mr Dowden continued: "A lot of what we do as firefighters and officers - obviously we have policies we refer to - but a lot of what we do and the instantaneous, that split-second decision-making, is based on previous knowledge.
"I had no previous knowledge on how that building was reacting, I had nothing to fall back on, no default in terms of my own previous knowledge about how that building was reacting at that moment in time.
"I did feel out of my comfort zone because I didn't have any previous experience to fall back on in terms of how that building was behaving and reacting."
The officer said he ordered a covering jet to be aimed at the fire when he began suspecting it had escaped through the flat window, at around 1.13am.
But the development did not prompt him to reconsider the stay-put advice to residents, he told the inquiry.
Most high-rise blocks in the UK are designed so that any fires are contained within the flat of origin - known as compartmentation - allowing a stay-put policy for other residents to be enforced.
But the flammable cladding installed around the west London tower fatally undermined this advice, as it allowed a blaze to engulf the structure within minutes.
Mr Dowden was asked if the fire spreading through the window could be seen as a failure of compartmentation.
He replied: "Not specifically, because, for me, the compartmentation is the internal part of the building.
"For me, as an incident commander on the outside, I wouldn't be too concerned if the fire breached the window because my assumption is we could tactically control that from the ground, particularly with a covering jet."
Mr Millett asked: "The exit of flames from the window - would that be a game-changer in your strategy?"
Mr Dowden said: "Not at that point - the only change in strategy we had in place was the implementation of a covering jet."