In general these sources are known employees of these agencies whom the journalist knows and can vouch for. However the idea is they would never give up their n

In general these sources are known employees of these agencies whom the journalist knows and can vouch for. However the idea is they would never give up their name. These same people may sometimes be quoted directly but we don't know. So the journalist believes the source that's why it is printed. If it turned out to be demonstrably incorrect then next time you don't trust the journalist and the journalist doesn't trust the source. Remember this is a 2 way street. The intelligence agencies want everyone to know it was them, but they can't go claiming credit in public,so you get an unnamed source.