BOA VISTA, Brazil (AP) — The scorching Amazon sun beats down as a group of agents inspect the body of a black helicopter. Nearby, in the backyard of the federal police headquarters in the city of Boa Vista, sit more than twenty aircraft — all seized.

Some bear signs of violent crashes: caved-in cockpits with wings broken off. Others feature interiors with stripped-out passenger seats in order to load up with more men and women, plus additional motors, fuel, food, and other cargo. Before they were confiscated, the aircraft were allegedly used for flying in and out of illegal gold mining sites.

Here in Roraima state, where all gold mining is illegal, they are essential for transporting prospectors and equipment to far-flung Indigenous reserves, including Brazil’s largest, Yanomami. Environmental and Indigenous rights groups estimate some 20,000 illegal miners are present on the reserve that is roughly the same size as Portugal. Government officials, including Brazil’s Vice President Hamilton Mourão, put the number closer to 3,500.

“Our focus over this last year has been to go after the logistics of illegal mining,” José Roberto Peres, the police superintendent for the state, told the Associated Press during an interview in November. “These are expensive machines; we can deduce that there is a lot of money involved.”

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