The FBI is "working with the Harney County Sheriff's Office, Oregon State Police and other local and state law enforcement agencies to bring a peaceful resolution to the situation,” the agency said in a statement.
The militia members who occupied the wildlife refuge buildings set up a roadblock, and two armed members had manned a guard tower that is usually used to spot wildfires. But there was no sign of law enforcement in the area, and local police said they had no intention of going to the scene, not even to keep watch on the militia.
The Rally and Occupation
The protest began Saturday as a rally in support of Harney County ranchers Dwight Hammond Jr. and Steven Hammond, who are to report to prison today for committing arson. The Hammond brothers left eastern Oregon early Sunday to report to Terminal Island in San Pedro, California, to serve their prison sentences.
"We all know the devastating effects that are caused by wildfires," Acting U.S. Attorney Billy Williams said. "Fires intentionally and illegally set on public lands, even those in a remote area, threaten property and residents and endanger firefighters called to battle the blaze."
After the rally for the Hammonds on Saturday, militia, along with sons of Cliven Bundy -- who was involved in a standoff with the government over grazing rights in Nevada in 2014 -- initiated the occupation of the headquarters of the Malheur National Wildlife Refuge.
Bundy's son Ammon claims the federally owned wildlife refuge in rural, eastern Oregon belongs to the people, and that they are "making a hard stand against ... overreach."
He said the government's "taking of people's land and resources" is leaving people in poverty, adding that the wildlife refuge "has been a tool in doing that."
Ryan Bundy and another of Ammon Bundy's brothers are also among the occupiers, according to The Associated Press.
Who Is Cliven Bundy and Why Is He So Controversial?
Ammon Bundy called the earlier rally successful, but said of the Wildlife Refuge standoff, "If we do not make a hard stand, we will be in a position where we won't be able to as a people."
He also asked for militia members to come help him.
Ammon Bundy says the group's actions are not aggressive and there is no damage or criminal activity.
He said the group's goal is to help local workers, including ranchers, miners and hunters, benefit from the land. The group wants to assert that the federal government does not have right to own or control land inside the state, Ammon Bundy said.
"We're prepared to be out here for as long as we need to be," he said in an eight-minute long Facebook video posted early Sunday morning.
The group does not have plans to occupy any other federal buildings, Ammon Bundy said Sunday.
The refuge is federal property managed by the U.S. Fish and Wildlife Service and was closed for the holiday weekend.
A U.S. Fish and Wildlife Service spokesperson told ABC News: "The Fish and Wildlife Service and The Bureau of Land Management have received reports that an unknown number of individuals have broken into the Malheur National Wildlife Refuge facility near Burns, Oregon. While the situation is ongoing, the main concern is employee safety and we can confirm that no federal staff were in the building at the time of the initial incident. We will continue to monitor the situation for additional developments."
The refuge headquarters was empty at the time of the seizure, Harney County, Oregon, Sheriff Dave Ward said in a statement.
"These men came to Harney County claiming to be part of militia groups supporting local ranchers, when in reality these men had alternative motives to attempt to over throw the county and federal government in hopes to spark a movement across the United States," Ward said.
"We are currently working jointly with several organizations to make sure the citizens of Harney County are safe and this issue is resolved as quickly and peaceful as possible," he said, adding that no other areas in Harney County are in "immediate danger."
"We ask that people stay away from the refuge for their safety," Ward said. "We also ask that if anyone sees any of these individuals in the area to please contact law enforcement and do not confront the individuals themselves."
Harney County School District No. 3 schools will be closed this week, Superintendent Dr. Marilyn L. McBride told ABC News.
"Ensuring staff and student safety is our greatest concern," McBride said.
Beth Anne Steele, a spokeswoman for the FBI in Portland, told ABC News the FBI is aware of the situation but is not making any further comments.
The Cliven Bundy Incident
Cliven Bundy, the patriarch of a large Mormon family with more than 50 grandchildren, came into the spotlight in April 2014, when the federal government started impounding his 900 head of cattle, following a 20-year battle over cattle-grazing on federal land.
The government said Bundy owed $1.1 million in unpaid grazing fees and penalties for continuing to let his cattle roam free on land near Bunkerville, Nevada, 80 miles northeast of Las Vegas, even after the government established the area as a protected habitat for the endangered desert tortoise in 1993 and slashed Bundy's cattle allotment.
The situation escalated the week of April 5, 2014, as hundreds of supporters from around the country rallied on Bundy's property to protest the federal cattle round-up. The dispute reignited debate over Bureau of Land Management practices, especially in Nevada where federal agencies control 85 percent of the land.
The confrontation turned ominous as armed militia gathered on his cattle and melon farm, aiming semi-automatic weapons at armed BLM officials from a bridge overpass. Some protesters were tasered by authorities and others arrested and later released, including one of Bundy’s 14 adult children.
On April 12, 2014, the BLM ended the stand-off, returned Bundy’s confiscated cattle and left the land citing safety concerns.
What to Know About the Militia Movement
The occupation is essentially "the spill over from the Bundy stand-off" in Nevada, according to Heidi Beirich, Director of the Intelligence Project at the Southern Poverty Law Center.
"What we're really seeing is a continuation of what started in April 2014, of militia folks and anti-government folks deciding that they're not going to accept federal authorities over federal lands," Beirich told ABC News Sunday.
"At the Bundy ranch, the federal government stood down. They had absolute cause to take Bundy's cattle. The Bundys were able -- at the point of a gun -- to drive the federal government and its representatives ... off the land," she said.
"Bundy is still a free man. He hasn't paid his money, and it's emboldened the entire movement to basically think, 'We don't have to follow the rules,'" Beirich said, explaining that that is what's happening now in Oregon.
The Bundy incident in 2014, as well as another incident in Oregon last year, "enlivened" the militias, she said, because they made them feel successful.
"They made the federal government back down from enforcing the law," she said. "And that has emboldened all these people, giving life to the movement."
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The Associated Press contributed to this report.