The FBI has found “a trench of human feces” on or near a Native American burial ground at the federal wildlife refuge in Oregon that armed militia members had occupied for more than a month, Reuters reported Wednesday.

In a court filing obtained by Reuters, U.S. Attorney Billy Williams wrote that investigators, who started processing the crime scene two days after the standoff ended, found “significant amounts of human feces” in a trench “either on or next to a ‘sensitive cultural site'” at the Malheur National Wildlife Refuge in Burns.

“Occupiers appear to have excavated two large trenches and an improvised road on or adjacent to grounds containing sensitive artifacts,” Williams wrote in the filing, as quoted by Reuters.

Firearms and explosives were also found on site, according to the report.

The FBI said Friday that they are cooperating with the Burns Paiute Tribe to identify “artifacts and sacred burial grounds” at the refuge, which is located in the high desert of eastern Oregon.