Nasdaq OMX says hackers have breached its computer systems to gain access to highly confidential data on publicly listed companies.
The bourse confirmed a Wall Street Journal report that it had been targeted by hackers who broke into Directors Desk, a Web-based service that lets management and board members of listed companies share confidential documents "securely".
Nasdaq OMX detected "suspicious files" in its systems late last year. They were removed and the FBI New York cyber-crime unit and US Department of Justice were called in to investigate.
An anonymous federal official has told The Associated Press that the hackers broke into the systems repeatedly over more than a year.
No conclusions on the motive for the attack have been reached but theories include the use of non-public information to gain a trading edge, terrorism, theft and wire fraud.
On Wednesday Nyse Euronext shut its Directors Desk equivalent, eGovDirect.com, for technical reasons says the Journal.
Cyber threats are an increasing concern for exchanges. Last month the Times reported that the LSE and counterparts in the US have called in authorities to help fend off persistent computer attacks.
Meanwhile, the European Commission has been forced to suspend all emissions trading after hackers broke into systems, stealing credits worth millions of euros.