This source links to the SD form filed with the SEC by American Tower in 2015 - they usually include a Conflict Minerals Report, but may also be a statement of why the company is not required to submit a Conflict Minerals Report. Section 1502 of the Dodd-Frank Act makes submitting a Conflict Minerals Report mandatory for companies that manufacture products requiring Tin, Tantalum, Tungsten or Gold (3TG, or "Conflict Minerals").