An anonymous letter sent to Romney’s accounting firm and political offices in Tennessee and published online sought $1 million in hard-to-trace Internet currency to prevent the disclosure of his tax filings, which have emerged as a key focus during the 2012 presidential race. Romney released his 2010 tax returns and a 2011 estimate in January, but he has refused to disclose his returns from earlier years.

Romney’s accounting firm, PricewaterhouseCoopers, said there was no evidence that any Romney tax files were stolen.

“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,” PricewaterhouseCoopers spokesman Chris Atkins said.

In Washington, Secret Service spokesman Edwin Donovan confirmed the agency was investigating. The Romney campaign declined to comment, referring all questions to the accounting firm.

Franklin police said there were no recent alarms or break-ins reported at the site. “We’ve had nothing from that address in August,” Police Lt. Charles J. Warner said.

There was no sign of forced entry at the five-story building that housed the accounting firm’s local office, not far from the Cool Springs Galleria, a large mall about 20 miles south of Nashville.

The building does not restrict access during business hours and has no guard. After-hours access to the doors and elevators appear to be controlled by keycard. A spokeswoman for the building manager, Spectrum Properties, said the company would not speculate on the burglary claim.

“All of the tenants operate independently and the building is highly secured,” the spokeswoman, Beth Courtney, said.

The data theft was claimed in letters left with political party offices in Franklin and disclosed in several Tennessee-area newspapers. Jean Barwick, the executive director of the Williamson County Republican Party, said employees in the GOP office found a small package on Friday with a hand-written address. The package contained a letter and a computer flash drive, she said.

Peter Burr, the chairman of the county’s Democratic Party, said he received a version of the letter and a thumb drive on Aug. 27.

“I have no way of knowing this is real or not,” he said. “I doubt it is, but I suppose it’s conceivable.”

An anonymous posting on a file-sharing website said the returns were stolen Aug. 25 from the accounting firm’s office. After “all available 1040 tax forms for Romney were copied,” the posting said, flash drives containing encrypted copies of his pre-2010 tax records were sent to the firm and to Republican and Democratic party offices.

The group threatened to divulge the tax files by late September unless it was paid $1 million.

Barwick and Burr said they turned over the materials to the Secret Service.

“The agents said there wasn’t a whole lot they could say, but they agreed that bizarre stuff during campaign season isn’t exactly unusual,” Burr said.

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Braun reported from Washington. Associated Press writer Alicia A. Caldwell in Washington contributed to this report.