FBI monitoring Oregon refuge seized by armed men
The FBI is leading the response to the armed seizure of a US government building in Oregon, with officers trying not to provoke the men who have threatened violence.
The anti-government group seized the remote wildlife refuge on Saturday, saying it supported two ranchers facing jail time for arson.
Rancher Dwight Hammond, 73, and his son Steven, 46, have rejected the group.
The case has riled right-wing activists who resent government interference.
The father and son were convicted three years ago of setting fires on federal land in 2001 and 2006, which they said were set to combat invasive species and protect their land from wildfires.
They have served jail time for the previous offences, but a federal judge has more recently said their terms were too short, violating mandatory minimum sentences. They have been ordered to serve four more years each.
Mr Hammond and his son reported to prison on Monday, said a local police officer.
At the scene - James Cook, BBC News, Oregon
At the top of a metal observation tower two men surveyed the surroundings.
There was not much for them to see apart from satellite trucks, camera crews and a solitary, unexplained drone.
For miles around there was no sign at all of any law enforcement officers although the word in Burns, some 30 minutes away, is that the hotels are packed with FBI agents.
Critics of this occupation are putting pressure on the federal authorities to enforce the law and end it. They have accused the occupiers of this building of being domestic terrorists, using force to impose their will.
Those involved insist they are patriots and they claim they have the law on their side. The US constitution prohibits the federal government from owning this land, they say.
Those occupying the Malheur National Wildlife Refuge centre in Burns say they plan to stay for years and may use violence if police try to evict them.
While the ranchers' case has served as a rallying cry, the group says its ultimate goal is to turn over the property to local authorities so that it can be used free of federal oversight.
Among the group's leaders is Ryan Bundy, whose father was the subject of a separate high-profile armed stand-off with US authorities over the use of federal land that took place in 2014.
Taking a similar approach that led to a peaceful end in that incident, authorities are monitoring the takeover from a distance but avoiding provoking the group with any signs of force.
However, the strategy has drawn criticism, saying the restrained response is because the occupiers are white.
The FBI has said it wants "to bring a peaceful resolution to the situation".
Ryan Bundy told the Associated Press that he hoped the armed takeover would inspire others to seize federally owned land.
Federal land control has been a contentious issue for decades, and is rooted in the US's expansion westward.
Federal land ownership in the US
The US federal government owns large tracts of land that are located within state borders.
The land is usually held in trust for US citizens by entities like the US Park Service, US Fish and Wildlife Service, the US Bureau of Indian Affairs, US Forest Service, the US Bureau of Land Management, among several others.
Most federally owned land is concentrated in western states, with over 46% of land inside the state of Oregon owned by the federal government.
Other key states that contain large amounts of federally controlled land include California (35%+), Nevada (76%+), Utah (70%+), and Idaho (60%+).
The stand-off has stoked fears among residents in the remote town of Burns, located about 30 miles (48 km) south of the seized building.
One resident said he feared that the armed group - largely, if not completely composed of men from outside Oregon - may seek to harm the children of law enforcement officials who live in the area.
Mr Bundy has refused to say how many men were at the property, which is being guarded by pick-up trucks and armed men wearing camouflage.