The FBI said it has found a trench of human feces and a road excavated on or next to a sensitive cultural site with artifacts at the Oregon wildlife refuge where armed men staged a standoff with authorities, according to court records filed on Tuesday.

The filing came after the FBI on Friday said it was working with the Burns Paiute Tribe to identify damage to the tribe's artifacts and sacred burial grounds at the Malheur National Wildlife Refuge during the six-week occupation.

Evidence teams began processing the crime scenes at the refuge on Saturday, two days after the final occupiers surrendered, and the process will last about three weeks, according to the document submitted in Oregon federal court on Tuesday.

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U.S. Attorney Billy Williams of Oregon wrote in the filing that investigators found "significant amounts of human feces" in a trench at an outdoor camping area that was either on or next to a "sensitive cultural site."

"Occupiers appear to have excavated two large trenches and an improvised road on or adjacent to grounds containing sensitive artifacts," he wrote.

Williams also said in the filing - which was a response to occupiers' requests to have their attorneys allowed onto the site - that firearms and explosives were found, and it was feared vehicles and buildings could be booby trapped.

He said he would be willing to allow Bundy's legal team onto the site once authorities are finished processing it and before it is reopened to the public.

The takeover, which began on Jan. 2, was sparked by the return to prison of two Oregon ranchers convicted of setting fires that spread to federal property near the refuge.

It was led by brothers Ammon and Ryan Bundy, who were arrested in January along with other protesters on their way to speak at a community meeting in John Day, Oregon.

A spokesman for the group, Robert "LaVoy" Finicum, was shot dead during the stop.

The final four occupiers surrendered on Thursday with David Fry, 27, repeatedly threatening suicide in a dramatic final phone call with mediators before he gave up. All 12 people arrested in connection with the standoff will face charges of conspiracy to impede federal officers, according to the FBI.

The cost of the standoff will likely run into the millions of dollars, with local and state agencies looking to the federal government - and the arrested occupiers - to shoulder the bulk of the bills.