Authorities in Colorado have identified two causes that led to the most destructive wildfire in the state’s history. 

The Boulder County Sheriff’s Office concluded its 18-month investigation into the 2021 Marshall Fire, announcing June 8 that the fire had been formed from two separate fires. One of those started at a residential property where a legal burn that had been done six days earlier was reignited by high winds. The other fire, which ignited about one hour after the first, was caused by arcing power lines. Prosecutors have opted to file no criminal charges.

More than 6,000 acres burned. The fire destroyed more than 1,000 buildings—mostly homes—and damaged hundreds of others. Two people “and a significant number of pets” were killed, investigators wrote. About 35,000 people evacuated the area. State officials and insurers have estimated the damage costs at more than $2 billion. 

“The Marshall Fire was unlike any other wildfire in the history of Colorado in terms of impact on urban areas and the devastation caused,” investigators wrote in a report summarizing their findings.

Unusual weather conditions also contributed to the fire, investigators found. The area had an uncommonly wet and warm spring that year, which allowed significant grass growth, but then a warm and dry second half to the year which left wild grass “tall, dry and susceptible to ignition,” they wrote in the report. Then, Boulder County experienced extreme winds on the day the fire started, with a record 111 gusts exceeding 75 mph that day. The winds fed the fire and pushed it down the Marshall Valley toward the communities of Louisville and Superior. 

The legal burn had been covered with dirt, but the high winds reignited it, investigators say. The winds also caused a power line to disconnect and contact other lines, resulting in the arcing that started the second fire. 

Investigators said they were unable to determine whether coal seams burning underground also contributed to the second fire. While they found no evidence to confirm it, they also said that possibility could not be ruled out, either.