A wildfire that burned 71,000 acres in southern Utah this summer led firefighters and other officials to uncover several explosive-filled bunkers hidden near makeshift cabins illegally built by a survivalist on public land, police say.

A Thursday news release from the Iron County Sheriff’s Office detailed how firefighters discovered the first bunker June 27 — 10 days after the Brian Head Fire began.

While working to suppress the blaze, firefighters near Henderson Hill heard “popping sounds,” the release said. At first, they thought the popping was rocks exploding from the heat of the wildfire, but as the sounds continued for about five minutes, the crews realized it was ammunition exploding.

After the popping stopped, the firefighters hiked to the source of the sound and found a cabin that had burned to the ground. Nearby, the release said, they found a bunker dug into the ground, containing what appeared to be novelty hand grenades that had been altered by drilling out the bottom and plugging the holes with threaded, galvanized pipe plugs. They also saw what appeared to be explosive powder, fuses and ammunition.

The bunker contained a “large number” of boxes and containers used as food storage, the release said. The firefighters took photos and informed law enforcement.

The discovery impacted the firefighting. Authorities deemed the items potentially dangerous to firefighters and would allow only airplanes or helicopters to suppress flames in the area.

On June 30, a bomb squad from Washington County and the FBI removed the explosives from the bunker, while other officers contacted a person of interest at his home in Parowan.

The person said he owned the cabin and bunker and was responsible for the explosives there. He said he’d purchased, drilled out and threaded the hand grenades and insisted they weren’t an explosive hazard to officials in the area.

He also told officers there were seven or eight such structures — four of which were cabins with bunkers hidden nearby — throughout the area. Most contain explosives and food storage, and were built over the course of several years, he said.

The man is a “survivalist,” said Iron County Sheriff’s Lt. Del Schlosser. The man, Schlosser added, had been building and maintaining the structures for more than 30 years.

From the original bunker, officers found dozens of hand grenades — which were in an inert condition. Some had fuses and others did not, the release said. Bomb technicians destroyed several hand grenades at the site along with “a significant quantity” of black powder.

(Courtesy of Iron County Sheriff’s Office) The remains of an explosive-filled bunker found during the Brian Head fire in summer 2017.

The next day, investigators went to the other cabin and bunker locations and determined they had been built on public land managed by the U.S. Forest Service (USFS) and on land managed by the Utah School and Institutional Trust Lands Administration (SITLA).

Two of the cabins on USFS land had been destroyed in the fire. The release said they had been constructed by using concrete footings at the four corners of the cabin as anchors for the structure and were outfitted with a corrugated metal roof, a metal stove and chimney and some tools.

The last cabin on SITLA land was fully intact, about 4 feet wide, 8 feet long and 10 feet tall. It was made of plywood and two-by-four beams secured to the concrete footings. Inside, the cabins contained two bunk beds, food and water storage, reading material, comfort items and a “small amount” of ammunition, the release said.

At the sites, county and federal officials removed burned debris from cabins and all items from the bunkers, the release said. They have also removed burned items and debris from other small storage caches in surrounding areas that the person who built the cabins showed them. Some items had to be removed by helicopter, due to the remote locations.

Schlosser said the county is not aware of any other cabins, bunkers or caches in the area.

The government agencies on whose land the cabins had been built “take very seriously the danger explosive caches pose to firefighters and the public, as well as the degradation of the public land through the unlawful construction of shelters, bunkers and storage caches,” the release said.

The investigation into the illegal cabins, bunkers and storage caches is ongoing, the release said, and the person allegedly responsible has been cooperative.

Once the investigation is complete, the case will be turned over to the Iron County attorney’s office and the U.S. attorney’s office for consideration of charges and possible restitution.