A senior fire commander believed only one or two flats in Grenfell Tower would catch alight "if we were unlucky" after flames rose up the high-rise's exterior.
Incident commander Andrew O'Loughlin immediately recognised the fire's spread up the east side of the tower was "exceptional" but that it looked as though it had "possibly done its worst", shortly after he arrived at 1.55am.
He said his expectation was that the concrete building "should protect itself from the burning cladding", envisioning flames would enter only a couple of flats.
In a written statement to the Grenfell Tower Inquiry, he said:
Mr O'Loughlin, who has almost 30 years experience at the London Fire Brigade, estimated there were between 100-200 people still inside the burning building after being briefed on his arrival.
However, believing the fire would not systematically enter and then breach compartments within the block, he assessed it would probably be safer to keep them inside until the fire had been put out rather than risk evacuation down its single smoke-logged stairwell.
He later learned the brigade's policy advising those trapped to stay put and await rescue had been abandoned, concluding "there must be something happening that I didn't know about".
Some 71 people died in the fire on June 14 last year, with a 72nd resident dying months later.
The public inquiry into the disaster is currently hearing evidence from fire personnel during its first phase at Holborn Bars, central London.
Mr O'Loughlin said he and his colleagues should never have been put in the position where they were "having to take a reasonably high level of risk to rescue members of the public, whilst knowing that despite our efforts a large number of people were still going to die".