Greg Toppo

USATODAY

Corrections & Clarifications: A previous version of this story misstated the number of world leaders linked to offshore holdings. The documents identify 12 current or former leaders and 60 family members or associates.

A massive, anonymous leak of financial documents from a Panamanian law firm has revealed an extensive worldwide network of offshore “shell” companies — including ones with ties to Russian President Vladimir Putin — that allow the wealthy to hide their assets from taxes and, in some cases, to launder billions in cash, a German newspaper alleges.

The documents, combed through in the past year by hundreds of journalists worldwide, show links to 12 current or former world leaders, including dictators accused of looting their own countries.

The data reveal details of secret offshore companies linked to families and associates of Egypt's former president Hosni Mubarak, Libya's former leader Muammar Gaddafi and Syria's president Bashar Assad, among others. Also included: Ian Cameron, the late father of British Prime Minister David Cameron. The elder Cameron died in 2010.

It also alleges a billion-dollar money-laundering ring run by a Russian bank tied to associates of Putin. The bank allegedly channeled money through offshore companies, two of which were officially owned by one of the Russian president's closest friends.

"I think the leak will prove to be probably the biggest blow the offshore world has ever taken because of the extent of the documents," said Gerard Ryle, director of the International Consortium of Investigative Journalism (ICIJ), a non-profit organization based in Washington, D.C., which coordinated the reporting.

The cache is larger than the one leaked in 2013 by former CIA employee Edward Snowden — who tweeted on Sunday that the "Biggest leak in the history of data journalism just went live, and it's about corruption."

ICIJ said as much as $2 billion has been secretly shuffled through banks and shadow companies linked to Putin’s associates, and that the bank has been instrumental in building a network of offshore companies.

The “Panama Papers” cover a period from the 1970s to December 2015. Sueddeutsche Zeitung, a Munich-based newspaper, obtained the trove of documents more than a year ago from an anonymous source tied to the firm Mossack Fonseca.

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The newspaper has since shared the documents with more than 100 news organizations and 400 journalists in 80 countries, who have pored over them for the past 12 months. McClatchy Newspapers, one of the group's U.S. partners, noted that the list of billionaires investigated included 29 featured in Forbes magazine’s list of the world’s 500 richest people.

The investigative network Fusion noted that soccer fans "shouldn’t be surprised to learn" that several anonymous offshore entities using Mossack Fonseca are linked to FIFA, including a former FIFA secretary general, a former Union of European Football Associations (UEFA) president and FIFA presidential candidate, two "controversial Argentine sports and media marketers," and a Uruguayan lawyer who, "ironically, advises FIFA on ethical issues."

Among other revelations, the Mossack Fonseca documents show that Icelandic Prime Minister Sigmundur Gunnlaugsson and his wife bought an offshore company in 2007 that he didn’t declare when entering parliament in 2009.

The offshore company was used to invest millions of dollars of inherited money. Gunnlaugsson sold his 50% share of the company to his wife for $1 eight months later, the documents allege.

Gunnlaugsson, who is now facing calls for his resignation, said he has not broken any rules and that his wife didn’t benefit financially from the arrangement.

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Sueddeutsche Zeitung said Sunday that the 11.5 million-document cache, which includes 4.8 million emails and nearly 2.5 million documents, is several times larger than a previous cache of offshore data published by WikiLeaks in 2013.

Mossack Fonseca denied all accusations of illegal activity.

“We have not once in nearly 40 years of operation been charged with criminal wrongdoing," spokesman Carlos Sousa told McClatchy Newspapers. "We’re proud of the work we do, notwithstanding recent and willful attempts by some to mischaracterize it.”

The firm said it has always complied with international protocols to ensure the companies it incorporates are not used for tax evasion, money-laundering, terrorist financing “or other illicit purposes.”

"If we detect suspicious activity or misconduct, we are quick to report it to the authorities," the firm told BBC. "Similarly, when authorities approach us with evidence of possible misconduct, we always cooperate fully with them."

On Sunday, #PanamaPapers trended on Twitter as users around the world reacted to the disclosures.

Follow Greg Toppo on Twitter: @gtoppo