Former world 100m champion Tyson Gay tested positive for a banned steroid, the BBC has learned.

News of the American's failed test emerged last month, but the details of the substance he tested positive for have - until now - not been made public.

The BBC has obtained extracts of a letter sent to Gay by the United States Anti Doping Agency (Usada) which outlines the nature of the substance traced in a sample he gave to drug testers during the US World Championship trials in June.