What the Glenwood Springs Fire Department has unofficially dubbed the 111 Fire — after mile marker 111 on Interstate 70, near South Canyon and just south of where the wildland fire that broke out Wednesday afternoon — was 25% contained as night fell and aerial firefighting efforts were suspended due to lack of light.

Although the cause of the blaze is still under investigation, preliminary hypotheses suggest the fire was likely sparked accidentally by human action, according to Mina Bolton, acting public information officer of the incident and executive assistant for the Glenwood Springs Fire Department.

GSFD responded to a brush fire on federal land north of Interstate 70 mile marker 111 that broke out Wednesday at around 2:32 p.m.

Upon arrival, about half an acre was burning, with construction crews attempting to extinguish the fire, according to a city of Glenwood Springs press release.

“The fire spread quickly, growing to approximately 15 acres in a short amount of time,” it read.

By about 3:30 p.m., the fire had grown in size enough to initiate pre-evacuation orders in West Glenwood neighborhoods, including those around Ami’s Acres Campgrounds, according to the GSFD Facebook page — on which the department also urged residents to sign up for emergency communications via garco911.com. No homes had been evacuated Wednesday.

The Garfield County Sheriff’s Department, Carbondale & Rural Fire Protection District and Carbondale River Fire Rescue responded to the scene in a mutual aid capacity, with the former fire department supplying command staff to supplement GSFD Chief Gary Tillotson, who was not available for immediate comment.

Additionally, two Upper Colorado River Interagency helicopters — including a Type 1, which is the largest helicopter used on fires and carries between 700 and 2,800 gallons of water — and a small engine air tanker were deployed early as the primary firefighting tools, given the steep terrain of the area.

As of about 4:40 p.m., the fire had grown to between 40 and 50 acres, Roaring Fork Fire Rescue Deputy Chief of Operations Kevin Issel said.

“That’s just listening to the radio,” he said. “It could double in size in a matter of minutes and you don’t even know, unless you hear it on the radio. Forty to 50 acres is a significant fire.”

Official reports estimated the spread at about 40 acres, and as of Wednesday evening, the fire had been contained at between 35 and 40 acres.

At that time, it had only been the two Carbondale entities that have been called to assist with containing the blaze, but Issel expected that RFFR will also be called in if “it continues to explode.”

By about 5:10 p.m., an additional four aircraft were sent, bringing the total number of aerial firefighting units up to seven.

In response to the aerial firefighting, the interstate was closed in both directions during the day Wednesday. Westbound traffic on I-70 was closed from Dotsero (exit 133) until mile marker 109, west of Glenwood Springs. Eastbound traffic was closed from mile marker 109 to exit 114 (Glenwood Springs).

At 6:37 p.m., CDOT announced that eastbound I-70 traffic between New Castle and Dotsero had reopened with both lanes. Around the same time, GSFD updated its Facebook page to let drivers know that a single westbound lane on the interstate had also reopened, but "traffic is moving slowly."

Because these aircraft cannot fly at night, it was safe to reopen the highway in both directions, CDOT regional communications manager Elise Thatcher said.

"Aircraft were important to the firefighting efforts," she said.

Megan Tackett is a reporter for the Aspen Daily News. She can be reached at megan@aspendailynews.com or on Twitter @MeganTackett10.