FXCM Inc, an online foreign exchange broker, said its systems were hacked and a "small number" of unauthorised wire transfers were made from customer accounts, sending the company's shares down 25% to a record low.
All funds have been returned to the accounts that were compromised, the company said.
FXCM said it received an e-mail from a hacker claiming to have illegal access to customer information and that it had notified the Federal Bureau of Investigation.
An FBI spokeswoman said the bureau was "aware of the incident and is investigating."
FXCM said it was working with a cybersecurity firm to determine the scale of the incident and identify affected customers.
The company did not say when it was hacked or give any further details on the incident. FXCM did not immediately respond to a request for comment.
FXCM is the latest US corporation to become a victim of a cybersecurity attack, adding to a list that includes Target Corp, Apple Inc and JPMorgan Chase & Co.
Accusations against China have increased amid growing concerns about national security. Last week, US President Barack Obama and his Chinese counterpart Xi Jinping signed a pact to fight cyberwarfare. — Reuters
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