While authorities still don’t know what sparked the deadly fires in the Los Angeles area, they do know one clear way the flames have spread: embers.

At least 24 people have died in the fires that have destroyed more than 12,000 structures since starting last Tuesday. The flames have been fueled by strong winds, which not only aid combustion by increasing the oxygen supply but carry embers to unburned areas.

Contrary to popular belief, experts say most homes destroyed by wildfires aren’t overcome by a racing wall of flames, but rather burn after being ignited by airborne embers.

Here’s a look at what embers are and the role they play in wildfires.

What is an ember?

An ember is a piece of burning debris. Once it becomes airborne, the more technical term is firebrand, said James Urban, an assistant professor in the Department of Fire Protection Engineering at Worcester Polytechnic Institute.

“If it’s a wildland fire, it’s typically pieces of wood or other types of vegetation that are burning,” he said. “But when you have a fire that’s burning through an urban area, it can be vegetation, it can be pieces of the house, it can be almost anything that burns.”

They can range in size from tiny specks to larger chunks.

What are they capable of?

While many people might have seen stray embers rise from a campfire and even had one land on them, the embers involved in wildfires are drastically different, said Anne Cope, chief engineer at the Insurance Institute for Business & Home Safety.

“Those embers can travel for miles, and it’s often the neighborhoods that are closer to the wildlands that get inundated with just loads and loads — just showered with embers,” she said.

Wind allows embers to burn harder and release more energy, becoming a more potent ignition source, Urban said. The firebrands then accumulate and sort of work together, gathering between the slats of wood fences or in shrubbery and igniting new fires.

In 2017, embers blew across a six-lane highway in Northern California, igniting businesses and then jumping from house to house in the Coffey Park neighborhood in Santa Rosa.

“A common thought before that was, ‘We don’t think embers are going to get all the way across that interstate, that’s a far distance, they’ll never get there,” Cope said. Well, never is a dangerous word.”

A single ember that lands on the ground might burn out within minutes but can also smolder, Urban said. “And then a sudden change of conditions like wind gusts ignite flames and cause a lot of destruction,” he said.

How are researchers studying embers?

Together with San José State University, Worcester Polytech is part of a Wildfire Interdisciplinary Research Center. With funding from the U.S. Forest Service and National Science Foundation, researchers are examining how firebrands are produced and how that knowledge can be incorporated into models about how wildfires spread and defensive measures that can be applied to homes, Urban said. For example, his students have conducted experiments to see how vegetation management around buildings might affect how quickly a fire spreads between structures.

“I’m optimistic in a way that there’s a lot of research coming out of this and we’ll be better prepared in other fires,” he said. “We’re going to see more fires like this, and there’s going to be need to be changes if we want to change the outcome.”

Cope agreed.

“I would love to see us strategically incorporate preparedness for wildfire embers in far more areas than we currently do,” she said.