As CNN found, it isn’t easy for companies to avoid using cobalt from these smaller mines. Dealers at markets in the DRC were filmed buying cobalt without verifying its source and mining method. They then send it for processing where it is mixed with cobalt from other mines before ending up in batteries that power devices around the world.

BMW, Volkswagen, and Mercedes-owner Daimler acknowledged how difficult it is to verify the source.

BMW said it makes “every effort to ensure the highest possible standards in the labor practices of our suppliers” but admitted it could do more.

Apple (AAPL), which stopped sourcing from artisanal mines last year, is also reported to be in talks to buy cobalt directly from regulated mines in the DRC. It declined to comment on those reports.

Volkswagen — which has plans to launch dozens of new electric cars — has introduced new rules for all its suppliers to ensure there’s no child labor in the supply chain.