For the second time in roughly three months, JPMorgan Chase is scrambling to contain the fallout from a security breach of its vast computer network, according to several people with knowledge of the investigation.
JPMorgan, the nation's largest bank, recently found that hackers, with links to Italy or southern Europe, had gained entry to some of the bank's servers, these people said.
Read MoreJPMorgan:We are not aware of a new cyberattack
The discovery follows an attack that was uncovered in late July and suggests that it was more extensive than first thought. In that attack, hackers obtained entry to dozens of the bank's servers and reviewed information on more than one million customer accounts. Security experts briefed on the matter had said that the full extent of the July attack was not known and that it could take the bank months to discover all of the fallout.
"We are not aware of any new attack," Kristin Lemkau, a spokeswoman for JPMorgan, said. Any report that there was a second breach is false, she added.
It is unclear whether the latest discovery constitutes a second breach or is part of broader fallout from the first incident. The revelation illustrates the challenges for JPMorgan as the bank tries to secure all its systems against another threat.
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The original hack sent ripples through the financial system and prompted an investigation by the Federal Bureau of Investigation, even as Wall Street, which has been a frequent target for hackers in recent years, worked to guard against the threats.
The bank was also forced to update its regulators, including the Federal Reserve, on the extent of the breach. The bank said at the time that there was no indication that any customer money was taken and that it believed it had secured all its systems.
On Thursday, the bank's top executives, including Gordon A. Smith, who heads JPMorgan's community and consumer banking, were working to assess the extent of the latest attack, according to two bank executives.
NYT Correction: October 2, 2014
An earlier version of the headline with this article misstated the extent of the cyber security issues at JPMorgan Chase. While the bank found evidence of previously unknown hacking, it says the latest discovery does not constitute a breach separate from an earlier one.