The devastating Marshall Fire, which consumed more than 1,000 homes and killed two people, started as two separate fires on Dec. 30, 2021, according to an official investigation into its cause. Credit: John Herrick

After 18 months of exhaustive work, investigators with the Boulder County Sheriff’s Office, the District Attorney’s Office, the U.S. Forest Service, the Colorado Department of Reclamation, Mining and Safety and the Colorado Bureau of Investigation, have determined how the Marshall Fire started. 

The report puts an end to speculation about what caused the most devastating wildfire in Colorado history, one that upended the lives of thousands who are still working to rebuild their homes and lives. 

The immediate cause of the Marshall Fire was human beings — through their mistakes or possible negligence. Yet the underlying cause was a parched landscape from a changing climate and hurricane-force winds that could have transformed any spark into an uncontrolled wildland blaze. Though the district attorney said there isn’t evidence for criminal charges to be brought against the suspected responsible parties, there will no doubt be additional civil cases filed in the coming weeks and months. 

On Thursday, June 8, at the Boulder County Sheriff’s Office, the sheriff and district attorney presented to a room of gathered media the outcome of an investigation that, as the district attorney said, “left no stone unturned.” 

Though the investigation took thousands of hours, “I can confidently say we know what happened and why,” said Boulder Sheriff Curtis Johnson, who lost his home in the fire and struggled to keep his voice steady as he spoke about the fire’s impact.

The Marshall Fire, which consumed more than 1,000 homes, killed two people and possibly more than a thousand pets — and did more than $2 billion in damage — started as two separate fires on Dec. 30, 2021. The first, named in the investigators report as The Eldorado Springs Fire, started at 5325 Eldorado Springs Drive.

Six days before the Marshall Fire, residents of that address, who are members of the Twelve Tribes community — a fundamentalist religious group that owns the Yellow Deli on Pearl Street — started a fire of old fencing material, tree branches and pallets, among other “junk.”

Firefighters were called by those living nearby that day, but they had little concern at the time, the investigation said. Dec. 24 was cool and wet with almost no wind, and residents of 5325 told firefighters that after the fire burned down, its remnants would be buried with heavy machinery. The owner of the property later told detectives he did just that, but without adding water to the coals first — recommended protocol at the time for putting out private property fires. Since then, Boulder County has included adding water to local post-burn requirements.

At the press conference, Boulder County’s District Attorney Michael Dougherty stressed that not only did residents of the property not need a permit to have such a fire burning, but hundreds of such fires are conducted each year in Boulder County. This one just happened to be unlucky. Without the wind event almost a week later, most of us would never have heard of it.

The district attorney offered a metaphor to illustrate this lack of luck.

“Let’s say you have a frozen lake,” Dougherty said. “And hundreds of people go on the lake without a problem. It’s frozen. There’s no signs prohibiting people from going on the lake. On this particular day, a man takes a couple of kids out onto the ice. First responders come along and say, ‘The lake is frozen solid, you have nothing to worry about.’ At some point thereafter, one of the kids goes through the ice. Is the man responsible on some level? Of course. But was he criminally reckless? No.”

Buried embers can smolder for weeks, sometimes months. And just six days later, winds raging from the mountains breathed new life into embers that still hadn’t had time to extinguish. When firefighters arrived on Dec. 30, after a call from a homeowner nearby, they saw sparks flying from a partially covered slash pile, jumping into adjacent dry grass. Residents of the property were seen trying to put out the fire rising from the ashes of their previous burn. They were unsuccessful.

The shed on the 5325 Eldorado Springs Drive property that made its rounds on social media as one of the fire’s causes is not considered the cause of the fire, just another one of its victims.

In interviewing every one of the roughly 40 residents of the property, it was determined that no fire had been lit since Dec. 24, and no activity that would warrant a criminal charge had happened.

“There’s no way we could prove in court today that [the residents’] conduct on Dec. 24 was criminally reckless,” Dougherty said.

The second fire, called The Trailhead Fire, was most likely started by an unmoored power line managed by Xcel, the investigation concluded.

Though reports of a downed power line came directly after the fire, that line was determined to be a communications line without enough electricity running through it to start a fire. What did likely start the Trailhead Fire was a power line that had been disconnected by the extreme wind. It was flapping about in the breeze and making contact with other wires. Called “arcing,” this commingling with other power lines can — and did — create sparks that then fell into the waiting fire fuel of withered vegetation below.

As the sheriff said, the spring and early summer of 2021 was wet, encouraging ample growth of vegetation that proceeded to dry out as the summer and early fall became drought-like. Fire ecologist Adam Mahood, a post-doc at the USDA-ARS, Fort Collins, said a lack of early winter snow is making Boulder plants particularly fire-prone.

“If there’s no precipitation until Christmas, and it’s still 65 degrees out, you have all these plants that are going dormant because of the angle of the sun in the sky, and the vegetation is probably drier than it would be in mid-August,” he told Boulder Reporting Lab

So no matter how the sparks began, it is possible they would have caught hold. But that investigators determined Xcel-managed power lines caused those sparks means civil litigation is in Xcel’s future. Already, as of Thursday afternoon, suits are being filed against the energy provider claiming negligence.

It took some extra time to reach this conclusion, as the power line was reattached immediately following the Marshall Fire, a discovery that raised concern and questions. Initially, it was thought that such prompt repair might have been undertaken to thwart an investigation into the fire’s origin.

According to the investigation, interviews with Xcel employees revealed no evidence of attempts to mislead investigators. Instead, the investigation concluded that Xcel repaired the line because it was concerned about getting power back to its customers, as the days following the Marshall Fire were cold and snowy. And without power, pipes were liable to freeze, causing damage to even more homes in the area.

Dougherty said there was insufficient evidence to charge Xcel criminally. He said if he did bring charges against Xcel, he realized there would be cheers. “But that cheering would come to an abrupt stop in about six weeks” when a judge, he predicted, would dismiss the case due to lack of evidence.

He acknowledged, however, that civil suits have different evidence requirements. He also said that in his dealings with Xcel, “communication could have been more clear” regarding the difference in terminology between downed lines and “floating” lines — those that are disconnected but not touching the ground. That Xcel said there weren’t any “downed lines” in the ignition area was true when considering its terminology, but it did add an undue wrinkle to the investigation and raised suspicion.

Houses can been seen under construction in the burn area of the Marshall Fire that devastated Boulder County communities on Dec. 30, 2021. Credit: Don Kohlbauer

Heat from the coal seam not ruled out

While investigators are quite confident the Trailhead Fire was started by the disconnected power line, heat from Boulder-Weld coal field can’t be completely ruled out, they said. 

Mined between 1863 and 1939, the seam that runs under the Marshall Mesa Trailhead is burning even now, as it was during the Marshall Fire. And if it were the cause, the Marshall Fire wouldn’t be the first fire the seam started.

On Dec. 20, 2005, a surface vent reached roughly 373 degrees Fahrenheit, which was enough to light nearby grass on fire. Because of that fire, sections of the coal seam that were considered particularly risky were excavated and filled with “275 tons of material.” Since then, no surface temperature above 90 degrees has been recorded there. Meaning, it hasn’t been hot enough to start a fire.

There’s also a small possibility that embers from the Eldorado Springs Fire could have started the Trailhead Fire, but that’s also far from likely. The second fire was 2,000 feet from the first  (roughly the length of seven football fields) and slightly to the west. And as the path of the fire would indicate — and wind damage around Boulder County testified — winds whipped towards the east. So it’s improbable that even the most tenacious spark would fight upwind to start a second blaze.

Questions at the end of the press conference hinted at coming civil suits, as reporters asked if there could be a determination of where exactly the fires converged. Which fire burned which house? they seemed to ask. The sheriff and district attorney said that this investigation was about determining the fire’s origin, not its convergence. 

When asked about potential similarities to Pacific Gas & Electric, a San Francisco-based utility company that declared bankruptcy in response to a series of lawsuits filed against them for their equipment starting wildfires, Dougherty said Xcel’s situation is different.

“I think certainly our analysis and conclusions will play a role in civil litigation,” Dougherty told Boulder Reporting Lab. “Looking at PG&E, and looking at this [situation], one thing to consider is whether there were any defects or issues with maintenance, or substandard conditions maintained by Xcel in that area. And we didn’t find any defects in equipment or any deficiencies in the maintenance.”

Asked what comes next, the sheriff told Boulder Reporting Lab he’s focused on ensuring his team and the community is prepared for the next fire, because there will be a next fire, no matter how diligent we are about sparks from various sources.

“I’m not naive enough to think we will never have another fire in Boulder County,” Johnson said. “It’s highly probable that someday in this county we will have another fire that encroaches on an urban area. What we can do is educate our community to be prepared for that, and what they can do on their property to mitigate.”

No doubt, a long-running push to compel Xcel Energy to underground its power lines will surface again in the legislature and other political realms. 

This year, Boulder had a very wet spring, and just as Johnson said of 2021’s spring, the vegetation around Boulder is growing “thick and healthy and strong.” If that’s followed by months of dry conditions, we might have the ingredients for another catastrophe before the memory of Marshall has sufficiently dulled. As Brian Oliver, the City of Boulder’s wildland fire chief said, the time to mitigate your home’s risk is now.

“The thing that prevents the next Marshall Fire, or that catastrophic outcome, is driven by the homeowner’s side of the equation,” Oliver said. “If we can keep that home-to-home ignition from starting in the first place, that Marshall Fire scenario doesn’t take place.”

Tim Drugan is the climate and environment reporter for Boulder Reporting Lab, covering wildfires, water and other climate-related issues for Boulder with a focus on explanatory and solutions journalism. He also is the lead writer of BRL Today, our morning newsletter. Tim grew up in New Hampshire and graduated from UNH with a degree in English/Journalism. Email:

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  1. I find that our inability to prescribe burn the open space, proactively preventing large conflagrations through measured burning of the fuel load, was also a main contributor to the cause of the fire. There are intangible benefits to prescribed burning, such as species diversity, land regeneration, reducing excessive amounts of brush, shrubs, and trees, encouraging the new growth of native vegetation, maintaining the many plant and animal species whose habitats depend on periodic fire, increasing soil nutrients, controlling competition, reducing hazardous conditions, and improving wildlife habitat.
    Our native communities are demonstrating the wisdom in prescribed burns. Why aren’t we paying attention?

  2. I looked back at the the Dec. 24, 2021 weather. The cause of fire reports keep saying it was a rainy day. The mountains were getting snow but a Denver Channel Dec. 24, 2021 weather reported the “dry plains”. A weather chart reported .01 inches of precipitation, hardly a “rainy day” for the start of the Seven Tribes fire. Also, a Boulder County “Stage 1 Fire Restriction” was issued on November 30, 2021 which included East of 93. Why no discussion of the start of the Seven Tribes fire during a “Stage 1 Fire Restriction” and why wouldn’t the Fire Department not have taken this more seriously at the time due to that restriction? I don’t understand these discrepancies.

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