One of the founders of Mossack Fonseca, the law firm at the centre of the Panama Papers revelations on offshore holdings, said his company was hacked.
Ramon Fonseca said the leak was not caused by an insider but that the company was hacked by servers abroad.
The leaked documents from Mossack Fonseca showed how the world's rich and powerful are able to stash their wealth and avoid taxes.
The law firm is based in Panama and has more than 40 offices worldwide.
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"We rule out an inside job. This is not a leak. This is a hack," Fonseca, 63, told Reuters news agency at the company's headquarters in Panama City's business district.
"We have a theory and we are following it."
Fonseca said that the firm had lodged a criminal complaint with Panamanian prosecutors on Monday over the alleged server breach.
Fonseca also said that in all the reporting on the Panama Papers so far "nobody is talking of the hack, and that is the only crime that has been committed".
In a telephone message responding to questions from AFP news agency, Fonseca said: "We have lodged a complaint. We have a technical report that we were hacked by servers abroad."
He also rued the fact that reporting on the 11.5 million documents taken from Mossack Fonseca's computer system focused on the high-profile clients who had used the law firm to set up offshore companies to hold their wealth.
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"We don't understand. The world is already accepting that privacy is not a human right," he said.
Fonseca's comments came as a spokesperson for David Cameron said the British prime minister, his wife and their children would not benefit in future from any offshore funds or trusts.
Cameron's late father, Ian, was among the tens of thousands of people named in the leaked documents from Mossack Fonseca.
After having at first described it as a private matter, Cameron's office said on Tuesday that he and his family did not benefit from any such funds at present.
Cameron also said he did not own any shares or have any offshore funds.
But his failure to say whether he or his family would benefit in future only intensified media speculation, with the story splashed across many newspaper front pages on Wednesday.
"There are no offshore funds or trusts which the prime minister, Mrs Cameron or their children will benefit from in future," the spokesperson said on Wednesday.
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Cameron has cast himself as a champion in the fight against tax evasion, particularly in British-linked territories such as the British Virgin Islands and Cayman Islands.
However, the opposition Labour Party have said the Panama Papers show the government has failed to tackle the issue.
The Telegraph newspaper reported that Ian Cameron's fund moved its operations to Ireland in 2010, the year his son became prime minister, as the directors believed it was about to "come under more scrutiny".