CHALLIS, Idaho -- The Challis office of the Bureau of Land Management was completely gutted by fire overnight.

Sarah Wheeler, a public affairs officer for Idaho Falls district, said Tuesday morning that smoke was still drifting from the structure, and BLM employees have not been allowed inside. Although the siding-and-brick exterior walls remain standing, the roof is almost completely gone, and what can be seen of the building's inside is charred black.

"Initial reports say there's probably significant damage," Wheeler said.

It's unclear how the fire started. Wheeler said the Idaho State Fire Marshall and an arson task force has been called in to investigate, but added she did not want to speculate about whether the blaze may have been intentionally set.

Custer County Sheriff Stu Lumpkin was also at the scene of the fire Tuesday morning, his office said.

Firefighters who were called out to the BLM building before 2 a.m. "did a great job," Wheeler said, but it was too late: The fire had already extended into the roof and wreaked heavy damage inside.

"It's going to be uninhabitable," she said.

According to BLM officials, the Challis office manages 792,243 acres of public lands - mostly in Custer County - and employes 25 people.

Those employees have been told not to come in today, Wheeler said. The department is still figuring out logistics, but hopes to set up a temporary office so those employees can continue working, she said.

The scope of the loss of documents and computer files has not yet been calculated, and the effects of those losses remain to be seen.

"We're going to be looking for a strategy of where do we go from here," Wheeler said.