Residents of London’s Grenfell Tower should not have been advised to stay in their apartments as flames engulfed the 25-story building, according to initial findings of a public inquiry released on Monday.

The findings released on Monday are part of the second stage of the ongoing Grenfell Tower Inquiry, which looks into what caused the fire.

The first emergency call reporting the fire, which killed 72 people, was made just before 1 a.m. last June 14. The deadly blaze started in a fourth floor apartment kitchen, but within minutes the flames broke out of the apartment and spread to other floors.

Within 30 minutes, it had reached the 23rd floor, Barbara Lane, a fire engineer appointed by the public inquiry to investigate the cause of the fire, wrote in the inquiry report.

By that point, the advice to residents from London’s Fire Brigade to “stay put” had “substantially failed,” Lane wrote in the report. It wasn’t until nearly two hours after the fire started that the fire service’s “stay put” strategy was changed to an “all out” strategy.

Lane and other experts pointed to external cladding that had been fitted to the building during a refurbishment that took place between 2012 and 2016 as the main reason the fire spread so fast. A building’s cladding consists of external layers of materials to separate the interior from exterior elements like weather.

Once the cladding on the fourth floor caught fire, the flames spread seven floors within seven minutes and 19 floors within 12 minutes, Lane said in the report.

Since the outside of the building couldn’t resist the spread of the fire, the residents of the tower's other apartments “were not safe to remain, as they were increasingly being directly affected by heat and smoke due to the external fire,” Lane wrote.

But trying to escape was a dangerous prospect as well, experts said. Between 1:59 and 2:58 am, there was thick smoke between the 7th and 12th floor, according to the report. Residents described the lobby on the 10th floor as “incredibly hot” and the stairway as “boiling hot”.

“It is my opinion that the witness evidence about the conditions in the stair[way] and the lobbies is deeply disturbing, and for many appeared life-threatening from early in the fire event,” Lane wrote.