WASHINGTON (Reuters) - The Federal Reserve said on Thursday it was still working to determine the extent that its computer systems had been breached by hackers, adding that the incident was the subject of a criminal investigation by the Federal Bureau of Investigation.

"We are in the process of a comprehensive assessment to determine what information might have been obtained in this incident," said Federal Reserve spokesman Jim Strader. "We remain confident that this incident did not affect critical operations of the Federal Reserve," he said.

The online intrusion, which has embarrassed the Fed and raised questions about the effectiveness of its security, was publicized on Sunday by activist group Anonymous.

(Reporting by Alister Bull)