

The Panama Papers investigation into the offshore secrets of the rich and famous has been awarded a prestigious Pulitzer prize.

The prize was given to the Washington-based International Consortium of Investigative Journalists (ICIJ), together with two US titles, McClatchy and the Miami Herald.

The judges cited a series of stories from the Panama Papers and the “collaboration of more than 300 reporters on six continents to expose the hidden infrastructure and global scale of offshore tax havens”.

The consortium included Germany’s Süddeutsche Zeitung – which received 11.5m documents from a whistleblower – the Guardian, the BBC and about 100 other international media organisations. The project is believed to have been the largest journalistic cooperation ever.

The investigation was originally entered into the international reporting category where it was a shortlisted finalist. It was moved to the 2016 explanatory reporting category, which it won.

The German reporters who received the leak, Bastian Obermayer and Frederik Obermaier, paid tribute to “John Doe”, their anonymous source. He – or she – provided internal files and emails from the Panamanian law firm Mossack Fonseca, which specialises in setting up opaque offshore companies.

Biggest thanks to John Doe who made this all possible https://t.co/awQgnG9dPy — Frederik Obermaier (@f_obermaier) April 10, 2017

Obermayer recalled how the story started, in early 2015, when he received an anonymous message one evening on his laptop. The source introduced himself as “John Doe” and asked Obermayer if he was interested in data.

Hello. This is John Doe. Interested in data? I'm happy to share- this is how it started more than a year ago #panamapapers #craziestyearever — Bastian Obermayer (@b_obermayer) April 3, 2016

What followed was a thrilling and secret year-long journalistic collaboration in more than 80 countries. Süddeutsche Zeitung shared its material with the ICIJ, which in turn gave access to the Panama Papers to media partners around the world via secure platforms.

The name was a conscious echo of the Pentagon Papers, volumes of secret documents leaked in 1971 by Daniel Ellsberg which lifted the lid on the US war in Vietnam. The Panama Papers investigation revealed that the offshore system – much of it run from British overseas territories and crown dependencies – was far more extensive than previously thought.

Those who featured in the revelations included David Cameron, whose late father, Ian, ran an offshore fund, Blairmore Holdings Inc. The fund used a small army of residents in the Bahamas to sign paperwork, including a part-time bishop.

The Panama Papers were published last April. Downing Street refused to answer questions about Cameron’s tax affairs, saying they were a “private matter”. Eventually Cameron admitted he had owned shares in his father’s offshore fund, selling them for £31,500 just before he became prime minister in 2010.

Cameron’s director of communications, Sir Craig Oliver, later said he thought the prime minister might have to resign.

The story had a global impact. Iceland’s prime minister Sigmundur Davíð Gunnlaugsson was forced to quit after it emerged that his family had sheltered cash offshore. There were demonstrations in Argentina and a small war in Azerbaijan, initiated – some believed – to distract from revelations concerning the president and his daughters.

In China, censors blocked the words “Panama Papers” and jammed the website of the Guardian. In Russia, aides to Vladimir Putin fumed about a western “spy” conspiracy after it emerged that Putin’s oldest friend, the cellist Sergei Roldugin, had about $2bn flowing into a network of British Virgin Islands companies.

The founders of the Panamanian law firm, Jürgen Mossack and Ramón Fonseca, were arrested in February. They are currently in jail on suspicion of money laundering following a coordinated swoop by prosecutors across Latin America.