The Federal Bureau of Investigation is probing a possible computer hacking attack on J.P. Morgan Chase & Co., in what people familiar with the probe described as a significant breach of corporate computer security.

The extent of the hacking -- when it began, what it affected, and what the potential costs for the company -- weren't immediately clear.

The company and federal cyber investigators are in discussions as they pursue the apparent attack on the bank's computers.

"Companies of our size unfortunately experience cyberattacks nearly every day," said Trish Wexler, a J.P. Morgan spokeswoman, in a statement. "We have multiple, layers of defense to counteract any threats and constantly monitor fraud levels."

Write to Devlin Barrett at devlin.barrett@wsj.com and Emily Glazer at emily.glazer@wsj.com

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