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JPMorgan Chase has said that information such as names and addresses for 76 million household customers and seven million businesses was compromised in a data breach this summer.
But the largest US bank said that there was no evidence that critical account information such as account numbers, user identities or social security numbers were stolen by the hackers.
"The bank continues not to have seen any unusual customer fraud related to this incident," JPMorgan said in a securities filing yesterday.
"JPMorgan Chase customers are not liable for unauthorized transactions on their account that they promptly alert the firm to," the bank said.
JPMorgan said in August that it was cooperating with law-enforcement officials following reports that hackers believed to be from Russia broke into the bank's computer systems. News reports at the time said other banks besides JPMorgan were targeted in the attack.
In August, the Federal Bureau of Investigations acknowledged that it and the US Secret Service were 'working to determine the scope of recently reported cyber attacks against several American financial institutions'.
Yesterday, a person familiar with the FBI probe said the investigation 'has made significant progress'.
The New York Times reported that JPMorgan was hit by hackers for a second time in three months. The report, citing people familiar with the probe, said the bank found that hackers from Italy or another part of southern Europe accessed bank servers.
But a JPMorgan spokeswoman called the report 'false' and said that the bank is 'not aware of any new attack'.
The case follows other huge data breaches at retailers Home Depot and Target. The Home Depot breach affected 56 million customer payment cards, while the Target breach affected credit card data for 40 million customers, plus personal and identification information for 70 million customers.
Other recent breaches have hit supermarket chain Albertsons, online retailer eBay, and hospital manager Community Health Systems.
US Treasury Secretary Jacob Lew and other top officials have highlighted cyberattacks as an increasing worry for financial institutions.
The issue has also spurred a new wave of investment, with cybersecurity companies like FireEye and Palo Alto Networks going public over the last couple of years.
