JP Morgan Chase bank, one of the largest financial institutions in the United States, announced last week that a massive computer hack affected the accounts of about 76 million households plus 7 million small businesses. This number of people and businesses suffered made it one of the largest hacks ever discovered.

The bank admitted that the attack lasted for almost a month before it was discovered in the middle of summer. The event was disclosed two months ago, and JP Morgan estimated at the time that about 1 million accounts had been compromised. However, the estimations proved wrong, as it turned out that the attack was much more serious than expected.

JP Morgan pointed out that the financial data didn’t leak, and that there had been no breach of login information, including account or SSNs, passwords or dates of birth. The hackers stole the names, email addresses, addresses and phone numbers of account holders.

Thus far, no unusual customer fraud using the leaked data has been recorded, and the bank clients won’t be liable for unauthorized transactions on their account, if they promptly alerted the bank. The largest bank in the United States by assets is cooperating with the Federal Bureau of Investigation and the American secret service to find the hackers.

This hack, one of the largest ever, occurred after a series of massive data breaches at various American institutions – for example, on Target and Home Depot. The latter has recently confirmed that its payment systems were breached in an attack which affected 56 million payment cards. As for Target, the last year attack has impacted about 40 million payment cards, while compromising the personal details of almost 70 million customers.

Nevertheless, the industry observers point out that the JP Morgan hack is much more serious, because the bank holds far more sensitive data than retailers. A few months ago, media reports accused Russian hackers of hacking JP Morgan and stealing gigabytes of sensitive data.

Apparently, the hack started in June and went unnoticed until July, so the criminals managed to access the accounts of over 90 servers. The suggestions were that the hackers originally entered JP Morgan’s systems after hacking into the computer of one of the bank’s employees. JP Morgan’s stock fell 0.89% in after hours trading following the news.