Calling himself "John Doe" -- a common legal 'placeholder name' for a person unknown -- the source offered to help authorities to bring tax cheats and other criminals to justice but voiced fears of meeting the fate of whistleblowers like Edward Snowden.
Writing in the German daily Suddeutsche Zeitung -- to which the original documents were leaked -- 'John Doe' also stressed that "for the record, I do not work for any government or intelligence agency, directly or as a contractor, and I never have".
The source's manifesto starts with the words "income inequality is one of the defining issues of our time" and blames it in large part on the "massive, pervasive corruption" of a wealthy global elite, aided by lawyers, politicians and the media.
"I decided to expose Mossack Fonseca because I thought its founders, employees and clients should have to answer for their roles in these crimes, only some of which have come to light thus far," wrote John Doe in an English-language text published by the Munich daily Sueddeutsche Zeitung.
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In the manifesto, John Doe argued that "the collective impact of these failures has been a complete erosion of ethical standards, ultimately leading to a novel system we still call capitalism, but which is tantamount to economic slavery".
"In this system -- our system -- the slaves are unaware both of their status and of their masters, who exist in a world apart where the intangible shackles are carefully hidden amongst reams of unreachable legalese."
The source says that history shows how issues involving taxation and power imbalances have "led to revolutions in ages past" and predicts that "the next revolution will be digitised. Or perhaps it has already begun."
Unlike the media organisations involved, "I ... Would be willing to cooperate with law enforcement to the extent that I am able", wrote the source.