The federal Bureau of Alcohol, Tobacco, Firearms and Explosives issued an open letter to "All Federal Firearms Licensees" outlining the relationship between federal firearms regulations and state marijuana laws. The letter explains federal law codified at 18 U.S.C. § 922(g)(3) prohibits unlawful users of controlled substances from possessing firearms or ammunition. Federal law also makes it unlawful to sell firearms or ammunition to someone about whom there is reasonable cause to believe uses or possesses controlled substances, such as marijuana. Therefore, the letter concludes, it is unlawful under federal law for medicinal marijuana card holders to buy or possess firearms or ammunition, regardless of a particular state's laws.