WOODSTOCK, Vt.—The accounting firm that handled Mitt Romney's tax returns says it has no evidence hackers have gained access to the Republican nominee's financial records.

The Nashville City Paper reported late Tuesday that an anonymous user posted a message on the file-sharing website Pastebin claiming to have hacked into the network of a PricewaterhouseCoopers office in Franklin, Tenn., and gained access to Romney's old tax returns. The alleged hacker requested $1 million to keep the supposedly compromised records sealed—but suggested he or she would turn over the records to anyone willing to pony up that same amount.

The Tennessean reported Wednesday that the Williamson County GOP in Franklin received an anonymous package containing a flash drive claiming to include a copy of Romney's taxes. The sender asked for $1 million to keep the records quiet.

But in a statement issued Wednesday, PricewaterhouseCoopers told reporters it had no evidence that its network had been accessed or that Romney's records had been tampered with.

"We're aware of the allegations that have been made about improper access to our systems," the firm said in a statement issued to the media. "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."

A Romney campaign spokeswoman declined to comment. But if the allegations are true, it will mark the second time this year the GOP presidential contender has had his personal information hacked.

In June, an individual contacted Gawker and claimed he or she had accessed Romney's personal Hotmail account—which immediately prompted a Secret Service investigation.