The US Secret Service said Wednesday it had opened a probe into the possible theft of copies of Republican White House hopeful Mitt Romney's tax returns.
"I can confirm that we are investigating the incident," Secret Service spokesman George Ogilvie said. Accounting firm PricewaterhouseCoopers (PWD) has said it is cooperating with the investigation.
The probe was triggered when an anonymous ransom note was posted online on the data sharing website Pastebin, demanding payment in exchange for the Romney tax data allegedly stolen from a PWC office in the US state of Tennessee.
The message asks for a ransom of $1 million to be converted into Bitcoins, a virtual currency, and sent to a specific address by September 28. In exchange, the tax returns of Romney and his wife Ann would be kept secret.
If the amount is not paid, the note warns, the tax data will be distributed to the media. The anonymous would-be thief also advises "other interested parties" that if they pay up first, they will be given access to the data.
The Republican challenger, who co-founded private equity firm Bain Capital on his way to amassing an estimated $250 million fortune, has defied mounting pressure from the Obama campaign to release more than two years of tax returns.
Romney's 2010 tax returns and an estimate of his 2011 returns reveal that over those two years he and wife Ann gave more than $4 million to the Mormon Church, 10 percent of roughly $40 million of disclosed income.
Mormons are expected to tithe at least 10 percent of their annual income to the church.
On its Twitter account, PWC said: "We are working with the Secret Service. At this time, there is no evidence of unauthorized access to our data."
Explore further: Social network challenges end in tragedy