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By Erik Ortiz and The Associated Press

The last four people continuing an armed occupation of an Oregon federal wildlife refuge have been indicted by a federal grand jury.

Ryan Bundy, Ammon Bundy, Brian Cavalier, Jon Ritzheimer, Peter Santilli, Shawna Cox, Ryan Payne and Joseph O'Shaughnessy are seen in a combination of police jail booking photos. Multnomah Co. and Maricopa Co. Sheriff offices / via Reuters

Sixteen people in all were charged with one count of conspiracy to impede officers of the United States, including the group’s leaders, Ammon Bundy and his brother Ryan.

The Bundys were arrested on Jan. 26. During those arrests, one occupier was shot dead. Eleven of the 16 indicted by the grand jury had previously been arrested on federal charges related to the occupation.

Ammon Bundy, through his attorneys, has repeatedly urged the four remaining at the Malheur National Wildlife Refuge to leave.

The armed occupation of the refuge near Burns began on Jan. 2. The group’s stated goals were the release of two ranchers ordered back to prison for burning public land, and that the federal government turn over public land to local control.

The occupation lost momentum after the FBI and state police arrested the Bundys and other leaders on a highway on Jan. 26, more than three weeks after the occupation began.

Related: Shawna Cox, Arrested With Oregon Militants, Released From Jail

One member of the group, Robert "LaVoy" Finicum, was killed by a state trooper after authorities said he reached for a handgun in his pocket.

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The remaining occupiers are: David Fry, 27, of Blanchester, Ohio; Jeff Banta, 46, of Elko, Nevada; and Sean Anderson, 48, and Sandy Anderson, 47, a married couple from Riggins, Idaho.

Kenneth Medenbach, who was arrested Jan. 15 in Burns while driving a U.S. Fish and Wildlife Service vehicle seized during the occupation, was also charged with conspiracy, Ammon Bundy's lawyer, Lissa Casey, said Thursday.

Fry's father, William Fry, was disappointed that his son has been charged, saying in an email: "We were hoping for a more positive outcome."

He told The Associated Press that his son is no different than those who believe "our country is heading in the wrong direction."

"He believes it so strongly he is willing to stand up with the hope that these actions might wake the nation up and lead others to get involved to change our country's course," the father said.

The indictment, which was handed down Wednesday and unsealed Thursday, alleges that the defendants conspired together since last October to prevent refuge employees from doing their work at the wildlife sanctuary "by force, intimidation, and threats."

On Oct. 5, two of the alleged conspirators went to Harney County, where the refuge is located, to warn the county sheriff of "extreme civil unrest" if certain demands were not met, the indictment says.

Then, on Jan. 2, the group occupied the refuge "by force while using and carrying firearms," and refused to leave or allow officials back in, according to the indictment.

The federal charge carries a maximum sentence of six years in prison.

The Bundy brothers — sons of Nevada rancher Cliven Bundy, who held a standoff with federal agents in 2014 over $1 million in unpaid grazing fees — remain jailed.