By Danny Yadron And Emily Glazer 

J.P. Morgan Chase & Co. may not have discovered the breach in its computer systems as quickly this past summer if it hadn't gone looking for trouble elsewhere, people briefed on the investigation said.

The bank learned hackers stole contact data for 76 million households and 7 million small businesses because the intruders had also used some of the same servers to hack both a corporate charity race website, whose breach was discovered first, and the bank.

The previously unreported account of the incident shows that J.P. Morgan was both ahead of the curve and behind it while investigating its massive data breach. On the one hand, the bank's investigators discovered the incident on their own by looking outside their sprawling network. On the other, the hackers were in the bank's network for months undetected.

In early August, a security vendor announced he had located a massive trove of email addresses and passwords that hackers had amassed from thousands of websites over the years. Buried in that cache: an indication the hackers hit the website for the J.P. Morgan Chase Corporate Challenge, a series of charity running races, people briefed on the investigation said.

J.P. Morgan and its security vendors discovered the cache included user information for several participants in the Corporate Challenge, which is managed by an outside vendor and isn't connected the bank's network.

Investigators linked that breach back to several overseas I.P. addresses. They queried J.P. Morgan's own network logs to see if there had been any communication with these addresses.

They found the servers had also been communicating with the bank's network.

The bank began to realize hackers had been in its system since at least June. They ultimately linked the attack to 11 I.P. addresses that were distributed anonymously to other banks in mid-August. Those I.P. addresses, viewed by The Wall Street Journal, link back mostly to eastern Europe, including Russia. Other addresses could be linked to Egypt and Brazil, according to a search of public Internet records.

The Federal Bureau of Investigation, which is leading the probe, didn't immediately respond to a request for comment.

The investigation has been hampered, the people said, because hackers deleted many of the log files that tracked their movements through the bank's network.

Devlin Barrett contributed to this article.

Write to Danny Yadron at danny.yadron@wsj.com and Emily Glazer at emily.glazer@wsj.com

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