defense systems, the refurbishment of tanks. it's a long list. >> reporter: pyongyang exporting its deadly expertise for hard cash, even to mozambique's remote interior, bolstering military installations like this, the u.n. says, training elite forces for at least two years, military sources tell us. all of it under sanctions for more than a decade. so how do they keep their operations secret? the trail leads us to one of the busiest avenues. >> according to documents, this is the headquarters of the north korean trade emissary here. >> reporter: reviewed by cnn, the documents name a shadowy front company. in 2017, the u.n. revealed that it helped funnel at least $6 million in military contracts to pyongyang. >> hi, how are you, sir? >> reporter: some asians were living here. they left three or four months ago, says the property agent. nobody could tell us where they