(CNN) -- An anonymous group says it stole copies of GOP presidential nominee Mitt Romney's tax records and will not release them unless the company it stole them from pays $1 million.
The Secret Service said it is investigating, and the company said there is no immediate sign that any such theft took place.
"Using your office" in Franklin, Tennessee, the group tells PricewaterhouseCoopers in an online posting, "we were able to gain access to your network file servers and copy over the tax documents for one Willard M Romney and Ann D Romney."
It threatens to send encrypted copies to "all major news outlets" and warns, "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."
If the money is not received, "the entire world will be allowed to view the documents with a publicly released key to unlock everything," the group warns.
The group demands $1 million worth of the online currency Bitcoins. It also says that people who want the documents released can send money as well, and whichever side sends $1 million first will win.
Bitcoin is a digital currency not overseen by any government or bank. Various merchants accept the currency for goods and services.
PricewaterhouseCoopers, which offers tax services as well as auditing and more, tweeted that it is "working with the Secret Service. At this time, there is no evidence of unauthorized access to our data."
The anonymous postings say that flash drive copies of the stolen material have been sent to the company, as well as to the county Democratic and Republican offices, and that a scanned image of Romney's signature from the forms was included.
Jean Barwick with the Williams County, Tennessee, Republican Party, told CNN that her office found the package -- a padded envelope -- on Friday. The package "didn't' seem credible," partly because it said "for learders" instead of "leaders," she said. Inside were a letter -- one that has been posted online -- and a flash drive.
"I didn't put that in any of our computers," and no one has looked at the contents, she said. "I put it in the drawer."
She called state party officials, who were in Tampa, Florida, at the Republican National Convention at the time, she said.
Her office later reported the package to local police, and the Secret Service picked it up Wednesday, she said.
County Democratic Party Chairman Peter Burr told CNN affiliate WTVF that he also thought the package was a scam, and he "almost threw it away," but the Secret Service showed up Wednesday to collect it.
No one looked at the contents of the drive, he said. While he may be interested in Romney's financial background, "We have no more right than anyone else to obtain information like that through inappropriate channels," he said.
Romney campaign spokeswoman Amanda Henneberg directed questions to PricewaterhouseCoopers.
While the postings, on the website pastebin.com, refer to "a team" involved in the alleged break-in, one ends with a line stating that certain "considerations did not deter me from the path of duty" and a reference to "the will of my Heavenly Father."
The postings go into detail about how the alleged heist was carried out.
The group says it obtained "all available 1040 tax forms for Romney," including some from before 2010, but it does not say which years.
Romney has released his 2010 and 2011 tax returns, and has said he will not release others. The issue has been a source of controversy on the campaign trail.
Pastebin allows users to paste text anonymously for a period of time. The company explains on its website that it was created to help programmers, and anything not related to that "which results in unusually high traffic will be flagged for investigation. Your paste may be deleted and your IP blocked. In particular, please do not paste email lists, password lists or personal information."
The company did not immediately respond to an e-mail from CNN asking whether it plans to remove the posts.