Bradley Birkenfeld, a banker-turned-whistleblower who exposed fraud in the Swiss banking system back in 2008, has said he believes the hack that led to the release of the so-called Panama Papers was orchestrated by a US intelligence agency.

The massive data leak, which resulted in over 11 million internal documents being released to the global media, outlined how law firm Mossack Fonseca aided a slew of wealthy people to set up offshore companies, some of which may have been used in tax avoidance schemes. While it is widely believed that a 'hack' on the firm's email server was carried out by a whistleblower, Birkenfeld said the lack of American names in the subsequent coverage is an indication of more "sinister" motives.

"The CIA I'm sure is behind this," he told CNBC in an interview from Munich, Germany. "The very fact that we see all these names surface that are the 'enemies' of the United States – Russia, China, Pakistan, Argentina – and we don't see one US name. Why is that?" he asked. "Quite frankly, my feeling is that this is certainly an intelligence agency operation."

He added: "If you've got NSA and CIA spying on foreign governments they can certainly get into a law firm like this. But they selectively bring the information to the public domain that doesn't hurt the US in any shape or form. That's wrong. And there's something seriously sinister here behind this." Asked why any agency would intentionally leak damaging information relating to a US ally – referencing the recent woes of UK prime minister David Cameron – Birkenfeld said he was likely "collateral damage" in a larger intelligence operation.

Birkenfeld was awarded more than $100m (£70m) by the US Internal Revenue Service (IRS) after disclosing evidence of financial fraud relating to how wealthy Americans were using the Swiss banking system to evade tax. The subsequent investigation against Swiss bank UBS, Birkenfeld's former employer, resulted in a $780m fine.

Amid an onslaught of media coverage, Mossack Fonseca director Ramon Fonseca has denied any wrongdoing. He said the firm had suffered a hack on its database and described the leak as "an international campaign against privacy", according to Reuters. "We rule out an inside job. This is not a leak. This is a hack," Fonseca said. "We have a theory and we are following it. We have already made the relevant complaints to the attorney general's office, and there is a government institution studying the issue." All of those implicated in the ICIJ Panama Papers report have been afforded the opportunity to respond: visit the ICIJ website to read the responses.