and sold for 20 to $30 million on the black market. all of that from a small container that looks like this. >> the vast majority is produce ed in china. it comes into the u.s. two ways. comes into mexico where they are combined with heroin. the other way is american consumers buying it direct, oftentimes from vendors out of china? >> it gets mailed in? >> u.s. mail, very common, a small quantity of fentanyl is hard to detect in the masses of letters that come into the u.s. every day. >> how effective is a wall at preventing drugs from getting into the united states. >> in terms of hard narcotics, don't know we get -- right now, the vast majority of hard narcotics come in through the parts of entry, in deep concealment, or through the mail order express consignment. >> it is worth pointing out again that that bag of flour, that one kilogram, two pound bag