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An aerial shot of burnt homes
Foundations are all that’s left of the Sagamore neighborhood in Superior after the Marshall Fire tore through the development last month. Every home in the development was destroyed in the fire. (Mike Sweeney, Special to The Colorado Sun)

Boulder County officials have completed their investigation into the Marshall fire — the most destructive in Colorado history.

Boulder County Sheriff Curtis Johnson and 20th Judicial District Attorney Michael Dougherty will address the outcome of the nearly 18-month probe into the cause and origin of the fire during a news conference scheduled for 10 a.m. Thursday. 

The update is likely to draw national and international attention. 

The fire, which broke out Dec. 30, 2021, burned more than 1,000 homes and businesses as wind-fueled flames raced across Superior and Louisville after sparking near Marshall Road and Colorado 93. 

Open burning was banned the day of the fire, which saw winds gusts upward of 100 mph. 

Federal agencies, including the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives, quickly joined forces with the Boulder County Sheriff’s Office in an attempt to pinpoint what caused the fire. Investigators followed up on more than 200 tips and reviewed footage from 266 body camera recordings and 400 photos that were captured by first responders during the initial response to the fire, the sheriff’s office said. The sheriff’s office also collected 49 physical items and 137 pieces of digital evidence, including drone footage and video recorded by witnesses. 

Officials have so far remained tight-lipped on details of the investigation. 

A search warrant was executed days after the fire on private property to determine the fire’s cause, but then-Sheriff Joe Pelle declined to release details on the location or name the property owner. Pelle, who was term-limited, left office in January after serving 20 years as the county sheriff. 

In April 2022, a lawsuit filed against Xcel Energy said its power lines were a “substantial factor” in the cause, origin and continuation of the fire. It said witnesses saw a fire igniting near a power line in the area where authorities say the fires started. The next hearing in the case is scheduled for July 11. 

During fire investigations, authorities consider a range of potential causes including lightning, cigarettes thrown from cars, campfires, underground coal mine fires and debris burning, the sheriff’s office said. 

It took just over an hour for flames to make their way from Colorado 93 east of Eldorado Springs to Louisville. People in the fire’s path were caught off guard and some barely escaped. Two died

Flames, fanned by wind gusts up to 110 mph, forced about 37,500 people to flee the area in about three to four hours, according to an after-action report, which called the effort “unprecedented.” 

Olivia Prentzel covers breaking news and a wide range of other important issues impacting Coloradans for The Colorado Sun, where she has been a staff writer since 2021. At The Sun, she has covered wildfires, criminal justice, the environment,...