The U.S. Secret Service is investigating a claim that tax records of Republican candidate Mitt Romney have been stolen and are being held for a $1-million ransom.

Accounting firm Pricewaterhouse Cooper confirms that it is working with the Secret Service regarding the report, but that "there is no evidence of unauthorized access to our data."

An anonymous note posted on Pastebin claimed that the tax records of "one Willard M Romney and Ann D Romney" were obtained during a break-in at the accounting firm’s offices in Franklin, Tenn., near Nashville.

"All major news media outlets are going to be sent an encrypted copy of the most recent tax years that your company had on file.… The years before 2010 will be of great interest to many. If the parties interested do not want the encrypted key released to the public to unlock these documents on September 28 of this year, then payment will be necessary.

"The deal is quite simple. Convert $1,000,000 USD to Bitcoins.… Transfer the Bitcoins gathered to the Bitcoin address listed below."

Bitcoin is an encrypted internet currency that defies tracing.

A statement from Pricewaterhouse Cooper says, "We are working closely with the United States Secret Service, and at this time there is no evidence that our systems have been compromised or that there was any unauthorized access to the data in question."

Romney, whose personal fortune is estimated at $250 million, has refused to release his tax returns for the years prior to 2010 and has drawn criticism for his elaborate accounting measures to avoid paying taxes.

Manila envelopes containing a printed copy of the threat and a USB stick were delivered to the Democratic and Republican party offices in Tennessee, reported CNET.

Peter Burr, chairman of the county Democratic Party, told CNET that the package was handed over to the Secret Service, but that he considered it "probably a scam."