Mike Snider

USA TODAY

Hackers inflitrated the security systems of the nation's largest bank, JPMorgan Chase, months ago and went undiscovered until a recent routine scan.

These latest revelations suggest that the hackers back in June took advantage of a security flaw in "the digital equivalent of JPMorgan's front door," Bloomberg reported, citing people familiar with the investigation.

Once inside the corporate network, the hackers used sophisticated programs to steal gigabytes of information, including customer-account data, until a routine scan performed recently triggered an alarm.

JPMorgan, which is working with the FBI and other authorities to determine the scope of a hacking attack that hit it and several other financial institutions, said it is not seeing unusual fraud activity.

Should customers detect any suspicious activity on their accounts, they should contact the bank, bank spokesman Michael Fusco said. "As we learn more, we will contact anyone we determine may have been impacted by this," he said Thursday.

JPMorgan CEO Jamie Dimon and other banking executives have been vocally concerned about increasingly complex cyber-intrusions. "The attacks are not new, they have been going on for years, but the frequency and the sophistication are increasing," said John Gunn, vice president of corporate communications for VASCO which provides security to the banking industry. "The fact that we don't see this type of story every day underscores that banks are very successful in stopping almost all of them."

Contributing: Jessica Guynn