The militiamen took over the facility following a rally earlier in the day for Dwight Hammond, 73, and his 46-year-old son Steven, The Oregonian newspaper reported.
The ranchers, who already served minimal jail time on arson charges after lighting what they said was a controlled fire on their property that spread to government land, were ordered to return to prison after a judge ruled their sentences had been too lenient, according to local media.
The pair, who were accused by prosecutors of starting the blaze to cover up hunting violations, were sentenced to around four years each in jail on top of the time they had already served — three months for the father and a year for his son, according to local media.
Following the demonstration by around 300 people Saturday in the town of Burns, a group of protesters advanced on the federal reserve 50 miles (80 kilometers) southeast of the town, where wild horses, pronghorns (an antelope-like mammal native to North America) and other creatures roam free.
“After the peaceful rally was completed today, a group of outside militants drove to the Malheur Wildlife Refuge, where they seized and occupied the refuge headquarters,” Harney County Sheriff Dave Ward said in a statement emailed.
The offices were closed and empty of people at the time, according to The Oregonian.
Among those holed up in the building were Ammon and Ryan Bundy, sons of rancher Cliven Bundy, who rose to prominence in 2014 during a standoff with federal agents over cattle grazing rights in the neighboring state of Nevada.
Ammon Bundy told The Oregonian by phone that the protesters occupying the reserve, whom he said numbered up to 100, would not rule out violence if the police stormed the facility.
“We’re planning on staying here for years, absolutely,” he said. “This is not a decision we’ve made at the last minute.”
Ward, meanwhile, said multiple agencies were “working on a solution” and urged people to steer clear of the area.
Arson on federal land carries a minimum of five years’ jail time, local broadcaster KTVZ reported.
The elder Hammond has already spent a total of three months in jail, while his son spent a year, according to local media.
The Hammonds are to report Monday to a federal prison in California, which borders Oregon in the south, KTVZ added.