The International Consortium of Investigative Journalists, the source of the Panama Papers and Paradise Papers, amongst other leaks, has published an article explaining how to leak information. It says that ICIJ is often approached by concerned citizens who believe they have found an injustice that they would like ICIJ to investigate, but few know the first thing about becoming a whistleblower or how to provide information to journalists. Hence, it has produced a basic guide to leaking.
https://www.icij.org/blog/2018/09/our-who-what-why-leak-explainer/
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