London’s Grenfell Tower was made worse by government “green energy requirements” that allowed fire to rapidly engulf the building Wednesday, leaving at least 17 people dead and scores more wounded or missing.

While it’s unknown what sparked the fire, experts say that the cladding, or exterior insulation, created a chimney effect through which the fire rapidly spread upwards. The cladding was added to Grenfell’s exterior in 2015 as part of a $12.8 million retrofit.

“I have never seen a fire that has engulfed an entire building like this in a career of more than 30 years,” Matt Wrack, who heads the Fire Brigades Union, told The Telegraph.

“It could be that this is the quest for sustainability trumping other concerns,” echoed Dr. Jim Glockling of the Fire Protection Association.

“There has been an emerging body of evidence surrounding some of the materials being used and now we have an appalling demonstration of what can happen,” Glockling said.

The Telegraph noted that cladding “is used as an insulation to make buildings more sustainable to meet green energy requirements.” Some 30,000 buildings in the U.K. have been retrofitted with cladding to cheaply comply with green energy mandates.

Grenfell Tower became more energy efficient, but the space between the cladding and the building increased the potential damage from fires, leaving hundreds of residents at the fire’s mercy.

“There were explosions everywhere you looked, lots of bangs, blue gas coming out everywhere you looked,” Mickey Paramasivan told The Telegraph.

“About 12 floors up I saw three children waving from a window and then there was just an explosion and they disappeared,” he said. “They were three kids, they were banging on the windows, you could see their silhouettes and then bang, it just went up.”

Prime Minister Theresa May has called for a full investigation into what happened. Others have called for all cladded buildings to be inspected for fire hazards.

However, building residents and experts have warned about the fire risks of cladding for years. A blog post by the Grenfell Action Group in November 2016 warned that “only a catastrophic event” would bring attention to the building’s issue.

Government officials cautioned against the risks of cladding since at least 1999, according to The Telegraph.

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