Someone claims to have stolen years of Mitt Romney's tax returns from a Tennessee office of the financial firm PricewaterhouseCoopers, and the Secret Service is investigating what appears to be a ransom scheme.

The local Democratic and Republican parties in Williamson County, Tenn., where the PricewaterhouseCoopers office is located, both received packages, each containing a thumb drive and a letter outlining a competitive-bidding ransom scheme that appears designed to pit Republicans and Democrats against each other over the release of Romney's taxes.

The letters stated an intention to publish Romney's tax returns on Sept. 28, unless $1 million is deposited in a Bitcoin (Internet currency) account, according to Williamson County Democratic Party Chairman Peter Burr, who said both he and his Williamson County GOP counterpart received similar packages. The letters did not make specific claims as to how many years of tax returns had allegedly been pilfered.

The Secret Service took the package from him after the local GOP contacted authorities, Burr said. The Secret Service confirmed to ABC News that it is investigating the ransom scheme.

The apparent hackers presented "two options," Burr said. "If somebody wanted to prevent the publication of these tax records, they needed to put a million dollars into this Bitcoin account…that was to prevent them from going ahead and publishing it on September 28th. Then they went on to say that if you want to ensure that this is published before September 28th, you have the option of contributing a million dollars to this other Bitcoin account … and if you win the race … the other side will not be able to prevent the publication."

PricewaterhouseCoopers acknowledged the scheme, first reported by the tech website Mashable, in a statement to ABC News.

"We are aware of the allegations that have been made regarding improper access to our systems. 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," the firm said.