FRANKLIN, Tenn. — The Secret Service said Wednesday it is investigating the reported theft of copies of Republican presidential candidate Mitt Romney's federal tax records during a break-in at an accounting office in Franklin. Someone claiming responsibility demanded $1 million not to make them public.

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.