The European Union is investigating whether US exporters have been shipping biodiesel to Europe via other countries to avoid high tariffs.
The EU started imposing those tariffs last year after Brussels ruled the US fuel was being illegally subsidised and that exporters were dumping it on the European market.
Since then US exports to the EU have plummeted while sales from Canada and Singapore have soared.
European biodiesel producers had complained the region is being flooded with cheap imports, putting them out of business.
The European Commission is also checking whether US exporters had switched to diesel blends containing less than twenty percent biofuel which do not have to pay the high tariffs of more than 400 euros per tonne.