Hackers have blackmailed Pricewaterhouse Coopers after claiming to have cracked the company’s network and stolen a record of US presidential candidate Mitt Romney’s tax returns.
The unnamed hackers have demanded $US1 million ($A980,000) to prevent the documents being sent to news outlets on September 28.
Pricewaterhouse Coopers said there was no evidence it was hacked.
The hackers said the years preceding 2010 would be "of great interest to many" in an anonymous post on Pastebin.
"The deal is quite simple. Convert $US1,000,000 USD to Bitcoins using the various markets available out in the world for buying," they instructed.
"Transfer the Bitcoins gathered to the Bitcoin address listed below. It does not matter if small amounts or one large amount is transferred, as long as the final value of the Bitcoins is equal to $1,000,000 USD at the time when it is finished.
"The keys to unlock the data will be purged and what ever is inside the documents will remain a secret forever."
The Secret Service confirmed to the Huffington Post the agency was investigating the incident. The publication reported that the Romney campaign declined to comment, and referred questions to Pricewaterhouse Coopers.
Blackmail letters were sent to Romney's Republication Party offices, including one package containing a hand-written address and a USB drive.
The hackers said the documents would be released to anyone who paid the ransom. Cryptographic Bitcoin transactions were irreversible and notoriously difficult to trace.
In an extortion attempt in February this year, a hacker trying to blackmail Symantec for $US50,000 was set up in a law enforcement sting.
Officers had masqueraded as an employee at the anti-virus company to extract information from the hacker.