DENVER (KDVR) — The Marshall Fire happened nearly a year and a half ago on Dec. 30, 2021, but investigators say they believe its origins may actually date to nearly a week earlier.

It was determined that two separate fires created the Marshall Fire, Boulder County Sheriff Curtis Johnson said Thursday.

The first started six days before the massive blaze ignited at a residential property located at 5325 Eldorado Springs Dr. on Dec. 24. The residents there had conducted a fire to dispose of scrap wood and tree branches.

That fire was reported by a passerby, and firefighters from the Mountain View Fire Protection District and the sheriff’s deputy responded. It was a cool, damp day, and there was a pool on the property.

“Given that there was a large water source on the property and the plan to cover the coals with dirt, the fire department believed the fire was being conducted responsibly,” Johnson said.

The residents buried the remnants of the fire at approximately 5 p.m. that day, and Johnson said there was no indication between then and Dec. 30 of any additional fire after the coals and ashes were buried.

There was a shed on the property that “was initially investigated as a potential location for where the fire started,” Johnson said. “But after our investigators exhumed and recreated the remains of the shed, we learned that there was no electrical service to the shed and no indication that the fire started at that location.”

  • Marshall Fire origin presser 3
  • Two photos shared by the Boulder County Sheriff's Office at a news conference on June 8, 2023 show an Xcel wire that the department says likely had hot particles ignite a fire on Dec. 30, 2021 and then combine with another fire to form the Marshall Fire.
  • Two photos shared by the Boulder County Sheriff's Office at a news conference on June 8, 2023 show: (l) a residential burn on Dec. 24, 2021 that it says was extinguished and had embers reignite on Dec. 30 and combine with another fire to form the Marshall Fire, and (r) the location of the residential burn in comparison to the start of the second fire on a map.
  • FILE - Homes burn as a wildfire rip through a development, Dec. 30, 2021, in Superior, Colo. Authorities say they have wrapped up their investigation into what started the most destructive wildfire in Colorado history and will announce their findings on Thursday, June 8, 2023. (AP Photo/David Zalubowski, File)

Just after 11 a.m. on Dec. 30, a high wind event uncovered the previously extinguished fire, exposing embers to oxygen and blowing them into dry vegetation nearby. That’s when the vegetation caught fire and flames began to spread.

“Our investigators consulted with the Missoula fire sciences laboratory and learned that, under the right conditions, smoldering combustion can occur for weeks or even months when buried,” Johnson said.

The residents were unaware of the fire at first, but once they realized it was happening and spreading, they attempted to put it out. Johnson said they couldn’t keep up with the winds spreading the flames.

At the time that crews arrived on the scene the shed on the property was not burning, according to Johnson, but it did eventually burn as the fire spread east.

After that, the fire overwhelmingly spread east with the winds toward the Marshall community. Approximately an hour later, a second fire ignited just south of the Marshall Mesa Trailhead.

Johnson said that investigators first looked into the possibility that embers from the first fire set the second blaze, but determined that it was not likely that embers moved 2,000 feet southwest against the wind.

The investigators determined that hot particles discharged from an electrical arc from a damaged powerline were likely the cause of the second fire. As winds continued the two fires eventually merged.