Brazil's Public Labor Prosecutor's Office said workers were put in a "degrading" environment and had their passports and salaries withheld by a building company.
The ATRI also found that congestion caused the industry to waste 6.4 billion gallons of diesel fuel, creating more than $32 billion in additional fuel costs.
Amazon claims it has no obligation to bargain with the workers, because they are employees of companies that Amazon uses to deliver packages to customers.
Lawyers for the country allege that the tin, tantalum and tungsten is taken from conflict areas and then "laundered through international supply chains."
Mars is planning to have cocoa it purchases physically separated from products that come from other unverified sources at every stage in its supply chain.
Workers at two Amazon facilities near Atlanta, Georgia, and a delivery facility in Skokie, Illinois, outside of Chicago, also voted to authorize strikes December 15.