WASHINGTON (AP) — Assuming it's not a hoax, the purported theft of Republican presidential nominee Mitt Romney's tax returns has all the trappings of a high-tech whodunit: a politically themed burglary, a $1 million demand in hard-to-trace Internet currency, password-protected data and a threat to reveal everything in three more weeks. But can it be believed?

The Secret Service and FBI were investigating the case Thursday after someone claimed to have burglarized a PricewaterhouseCoopers accounting office in Franklin, Tenn., and stolen two decades' worth of Romney's tax returns.

The claimed theft, made in an anonymous letter sent to the accounting firm and political offices in Tennessee, has surfaced a critical moment during the 2012 presidential campaign amid the Republican and Democratic conventions. The ransom target in the case — Romney's tax returns — was carefully selected: Romney, worth an estimated $250 million, has steadfastly declined to make public more than one year's tax returns so far, and Democrats have sought to portray him as so wealthy he is out of touch with middle class voters.