ZURICH/GENEVA, Jan 9 (Reuters) - A hacker claiming to be behind a cyber attack on Banque Cantonale de Geneve, or BCGE, used social media on Friday to threaten to leak confidential data on the Swiss bank's clients if its demands for payment weren't met.
Judicial authorities said a Geneva prosecutor was handling a formal complaint lodged by the cantonal (state) bank on Jan. 6, but gave no details.
"A criminal complaint has been received from the Banque Cantonale de Geneve. An investigation is under way," Henri della Casa, spokesman for Geneva judicial authorities, told Reuters.
"These cases are very difficult, it is complicated to find the authors (of a cyber attack)."
In potentially the latest case of a breach of customer information at a financial firm, an anonymous person or group using the Twitter moniker Rex Mundi claimed it had hacked the Genevan cantonal bank's servers and downloaded more than 30,000 emails by Swiss and foreign clients.
The hacker included names, addresses and messages to the bank from two people it said were BCGE clients, and said the remainder of the data it had stolen would be make public later on Friday if it was not paid 10,000 euros ($11,779).
Neither the identity of the self-described hacker nor the client data could be verified by Reuters.
BCGE said on Tuesday it had "resisted" a cyber-attack, and that information intercepted from its servers was "in no way critical and of no great use or even obsolete".
A bank spokeswoman said on Friday the bank was maintaining its position, without elaborating.
The hacker's tactics play into fears of tax dodgers and cheats using hidden Swiss accounts that their identity could be exposed.
"We would like to wish a merry tax audit to all the non-Swiss account holders listed in the BCGE files," the Rex Mundi account tweeted on Friday, one of 61 messages that range from detailing demands, to criticizing the bank for lax technology security, and taunting clients who may be at risk.
BCGE is one of a host of Swiss banks to come forward under a government-brokered scheme for banks to pay fines for helping wealthy Americans avoid tax by offering hidden offshore accounts.
Last month, Switzerland charged a former computer analyst at HSBC's private bank in Geneva with industrial espionage and breaching the country's secrecy laws for passing confidential client data to foreign authorities.
$1 = 0.8490 euros Reporting by Katharina Bart in Zurich and Stephanie Nebehay in Geneva; Editing by Mark Potter