A newly detected lightning fire, extreme winds and near-zero visibility contributed to the conditions surrounding a June 27 fire entrapment that ultimately killed four wildland firefighters on the Utah-Colorado border, according to a preliminary U.S. Forest Service investigative update released Thursday.

The Serious Accident Investigation Team said five members of the Rifle Interagency Helitack Crew deployed fire shelters while working the Gore and Knowles fires. Glenwood Springs resident Emily Barker, along with Nick Hutcherson and Sydney Watson, died during the incident, while two other firefighters were evacuated with burn injuries.

A fourth firefighter, Nathan Matthews, died July 24 from his injuries. The fifth firefighter continues to recover, according to the report.

U.S. Sen. John Hickenlooper called Thursday for the Department of the Interior to conduct its own “swift and thorough investigation” into the incident, including whether recent changes to the U.S. Wildland Fire Service played a role.

“These families deserve answers, and we need to understand what went wrong so we can prevent future tragedies,” Hickenlooper said during remarks on the Senate floor.

Hickenlooper said the inquiry should not be about assigning political blame, but about identifying lessons that could protect firefighters as wildfires continue burning across Colorado.

“This isn’t about politics or playing gotcha,” Hickenlooper said. “With wildfires still raging across Colorado, we can’t afford to wait months to learn the lessons that could save lives now.”

The investigation is being jointly led by the U.S. Wildland Fire Service and U.S. Forest Service. The agencies stressed in the update that the findings are preliminary and subject to change as investigators continue reviewing the incident.

The initial group of Rifle Helitack firefighters was assigned to the Knowles Fire at 9:36 a.m. June 27. The Knowles and Gore fires were about 8 miles east of two other initial attack fires known as the Snyder and Jones fires.

At 12:32 p.m., a new fire was detected about three-quarters of a mile northeast of the Snyder Fire and began exhibiting extreme fire behavior, according to the report.

Satellite imagery later confirmed the fire was started by lightning. Firefighters operating that day initially believed it was a spot fire from the Snyder Fire, which  burned more than 30,200 acres before it was declared 100% contained on July 17.

Conditions that day included very low fuel moisture, dry vegetation and strong west-southwest winds estimated at 30 to more than 50 mph. The report states that winds were intensified by the terrain and later shifted southward.

Fire danger measurements, including the Energy Release Component and Burning Index, were between the 94th and 99th percentiles, indicating unusually severe burning conditions.

The fire continued spreading rapidly through the afternoon. Strong winds held smoke close to the ground, reducing visibility to nearly zero near the deployment site.

At 2:56 p.m., crew members dispatched to be picked up by helicopter. Two minutes later, they reported that the fire was near their location.

A helicopter was en route at 3:08 p.m. and was estimated to be about five minutes away, but was ultimately unable to land because of heavy smoke.

The firefighters deployed their emergency fire shelters, portable safety devices used by wildland firefighters as a last resort, at about 3:19 p.m. Evidence at the site indicates the final fire impact came from the southwest, according to the report.

The Serious Accident Investigation Team is reviewing dispatch records, lightning data, photographs, satellite imagery, preliminary reports and observations of the fire’s behavior.

The update does not include final findings or conclusions about the cause of the entrapment or the decisions made before the deployment.

The agencies said additional information will be released when appropriate after the full investigation is completed.

“Our focus remains on supporting the families, survivors, and the wildland fire community,” the agencies said in the report. “The dedication of these firefighters is remembered and honored.”