LONDON - London's fire commissioner said Thursday morning that it would be a miracle if any more survivors were found after the devastating high-rise fire that killed at least 17 people.

That death toll was up by five from the figure given on Wednesday, but authorities said many more victims were still expected.  

Fire commissioner Dany Cotton said on British television that authorities didn't expect to find anyone else alive after the blaze.

CBS News correspondent Jonathan Vigliotti reports the damage to the 24-story building is so severe that fire officials said it would take weeks to inspect each floor carefully.

The cause of the fire remains unclear, reports Vigliotti, as does how it managed to spread so quickly up the apartment building. Fire experts believe, however, that cosmetic cladding -- siding added to the building's exterior in the last two years -- may be largely to blame for the scale of the devastation.

Commissioner Cotton said it was too early to speculate on how the deadly blaze started. She said early Thursday that authorities had finally extinguished the last pockets of flame, and were trying to secure the edges of the building for a fingertip search.

London fire investigators have been painstakingly searching for more victims of the inferno that engulfed the public housing building in London's North Kensington neighborhood.

The fire that began around 1 a.m. local time on Wednesday morning also injured dozens, 18 of them critically, and left an unknown number missing. The cause of the blaze is under investigation, but a tenants' group had complained for years about the risk of a fire.

The Grenfell Tower building underwent an $11 million refurbishment in 2015-2016 that saw the new external cladding installed. It was part of a wider $73 million development of the extremely pricey Kensington neighborhood, which included the construction of new homes.  

Newspaper reports cited the planning documents for that neighborhood regeneration as stating that the cladding was chosen, in part, as a low-cost way to improve Grenfell Tower's external appearance to new neighbors in the expensive homes being constructed nearby.

Similar external cladding has been linked to the fast spread of deadly fires on other apartment buildings both in the U.K., and as far away as Australia.
Residents who spoke to CBS News have said they spent years complaining about fire safety concerns, including blocked exits in the building, but that the management company was never responsive. While the death toll remains unclear, the BBC reported Thursday, citing anonymous sources, that as many as 100 people may have lost their lives in the blaze.

More than 1 million pounds ($1.27 million) has been raised to help victims of the tragedy as volunteers and charities worked through the night to find shelter and food for people who lost their homes.

The Associated Press contributed to this report.