The U.S. Federal Bureau of Investigation said it is investigating media reports that several U.S. financial firms have been victims of recent cyber attacks.

"We are working with the United States Secret Service to determine the scope of recently reported cyber attacks against several American financial institutions," FBI spokesman Joshua Campbell said in a statement late on Wednesday.

He did not name any firms or give further details. A Secret Service spokesman could not be reached for comment.

JPMorgan Chase & Co was the victim of a recent cyber attack, according to two people familiar with the incident who asked not to be identified because they were not authorized to speak publicly about the matter. They declined to elaborate on the severity of the incident, saying JPMorgan was still conducting an investigation to determine what happened.

JPMorgan spokesman Brian Marchiony declined comment when asked about the attack.

“Companies of our size unfortunately experience cyber attacks nearly every day. We have multiple, layers of defense to counteract any threats and constantly monitor fraud levels,” he said in a statement.

Earlier on Wednesday, Bloomberg News reported that Russian hackers were believed to have carried out cyber attacks against JPMorgan Chase and another unnamed U.S. bank in mid-August, resulting in the loss of sensitive data. Authorities are investigating whether the breaches were linked to recent attacks on major European banks, the Bloomberg report said.

The New York Times reported late on Wednesday that the networks of JPMorgan and at least four other U.S. banks had been infiltrated in a string of coordinated attacks this month, citing four people familiar with the investigation.

The attackers stole large quantities of data, including checking and savings account information, though their motivation is not yet clear, according to the Times report, which said several private security firms have been hired to conduct forensic reviews of infected networks.

Reuters was not able to independently verify the details in the Bloomberg and Times reports.