BURNS, Ore. (AP) — FBI officials said Friday they haven't found any rigged explosives or booby traps at the national wildlife refuge in Oregon that had been seized by an armed group.

Authorities allowed a group of reporters to get closer to the Malheur National Wildlife Refuge, where the last four occupiers surrendered Thursday. The tour stopped short of the refuge itself.

The armed protesters had blockaded the road near the refuge using a government-owned heavy front-end loader and two pickup trucks. A group of tents and pickup trucks was clustered on a small rise far beyond the road barrier. Larry Karl, the assistant special agent in charge of the Portland FBI, said the tents made up the "shantytown" where the last four holdouts at the refuge spent most of their time.

Greg Bretzing, the special agent in charge of the Portland FBI, said investigators have begun their sweep of the property. Karl said that process is still ongoing, however, but they hoped to finish the safety sweep of the buildings and begin processing evidence sometime Friday afternoon.

There was flammable liquid and hazardous materials stored at the site before the armed takeover, Karl said, and the FBI had information that "certain materials" might have been brought to the refuge by the protesters.

"So until we are able to go through the refuge and see what conditions those items are in, that they're properly secured and stored, or what has been brought on that we don't know about," the evidence collection will have to wait, Karl said.

He said the FBI would work closely with the Burns-Paiute Tribe, archaeological experts, federal land managers and others while they process the crime scene

The holdouts who surrendered Friday were the last remnants of a larger group that seized the property on Jan. 2, demanding the U.S. turn over public lands to locals and exposing simmering anger over the government's control of vast expanses of Western range.

David Fry, 27, of Blanchester, Ohio; Jeff Banta, 46, of Elko, Nevada; and married couple Sean Anderson, 48, and Sandy Anderson, 47, of Riggins, Idaho; were arraigned Friday in Portland, Oregon, along with two of the other 21 people who have been indicted.

They pleaded not guilty and U.S. District Court Judge John Acosta scheduled a 21-day trial to start April 19. Such dates are often pushed back by months.

Fry, the last activist to surrender, appeared in an anti-suicide smock. He waved to Nevada assemblywoman Michele Fiore, who visited Burns to help broker the end of the standoff and promised the holdouts she would be in the courtroom.

Outside the downtown Portland courthouse, Fiore read a statement from jailed standoff leader Ammon Bundy and defended the occupiers, much to the delight of a small group of protesters. She took issue with a reporter who asked if she advocated for more armed occupations.

"I advocate free, peaceful assembly, and that is exactly what happened," she said. "Nobody pointed a gun at anybody."

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This story has been corrected to show the FBI special agent in charge of Portland's last name is Bretzing, not Bretzinger.

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DuBois reported from Portland, Oregon.

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DuBois reported from Portland, Oregon.