The Source of the Papers
Really, there are only two. One is a disgruntled former employee who, Snowden-style, amasses a treasure trove of sensitive information and then passes it on to the media. There are a couple of problems with that explanation so far. First, we don't know who that individual would be. To have that level of access to information covered by client-attorney confidentiality (one of the sacrosanct Western values, at least as far as Western propaganda is concerned) one would have to be a highly placed employee of Mossack-Fonseca, for example a member of its Information Technology department with amazing hacking skills or appropriate security clearance. No individual has so far come forward, the firm in question has not identified any such employees, and if such an individual exists, why not come forward? He/she would be an instant global celebrity, the polar opposite of Edward Snowden whose revealing of US criminal wrongdoing earned him what might be a lifetime in exile, several rounds of condemnation by a variety of "liberal" politicians in the US, and not a few credible death threats. Whoever did this has every incentive to go public, go on speaking tours, write books - in short, become a celebrity.
Comment: Further reading: U.S. sends 3000 tons of weapons and ammo to al-Qaeda in Syria; Russian sends aid to civilians