By Devlin Barrett, Emily Glazer and Kirsten Grind 

Federal investigators believe the same hackers who stole data from J.P. Morgan Chase & Co. computers this summer also plucked some information from Fidelity Investments, according to people close to the case.

It is unclear which Fidelity networks were infiltrated or what types of data were taken from the company, one of the nation's largest investment firms, but so far, authorities don't believe the breach compromised account information or was on the same scale as the customer-contact information taken from J.P. Morgan, the people said.

Investigators have said at least 13 companies were targeted by the unknown hackers, making it one of the most significant cyberattacks disclosed in the U.S. To date, only J.P. Morgan and Fidelity are known to have had data stolen. An unspecified number of the companies' systems had some contact from computer addresses linked to the hackers but didn't lose any data, according to people close to the investigation.

Fidelity is one of the largest mutual-fund firms in the U.S., with $1.7 trillion in assets under management. The privately held company that grew in prominence during the stock-market boom of the 1980s and 1990s also operates a large brokerage and is one of the biggest providers of retirement services such as 401(k) plans.

A Fidelity spokesman said the company "continues to deploy multiple layers of security to help protect the privacy of our customers. Through our continued cooperation with law enforcement officials, we can reiterate and have confirmed with the FBI that there is no indication Fidelity customer accounts, information, services or systems were affected by the recent attack that impacted [J.P. Morgan]."

It remains unknown who is behind the incident that now appears to have victimized two of the largest financial firms in the U.S. Several people close to the investigations see ties to Russian-speaking cybercriminals. However, it is unclear where the alleged hackers are exactly, people close to the investigation said.

The move comes as investigators continue to probe what happened in a summer cyberattack that took contact information for 76 million households from J.P. Morgan and has shaken confidence in the security of the nation's financial firms.

The breach also affected seven million small businesses that do business with J.P. Morgan. No sensitive account information, such as passwords or Social Security numbers, was taken, the bank has said, but contact information such as e-mails were pilfered, raising the possibility that some customers will be exposed to sham emails.

A variety of regulators and prosecutors are investigating or examining the matter, including the Federal Bureau of Investigation, National Security Agency, Department of Homeland Security, U.S. attorney's office in Manhattan and New York's Department of Financial Services.

Some state attorneys general are also in discussions to form a multistate group to examine J.P. Morgan's cyberattack this summer, people familiar with the matter said.

Offices of attorneys general including those in California, Rhode Island, Connecticut and Illinois are in discussions to join together and examine whether J.P. Morgan, and possibly other financial institutions, followed state disclosure laws on data breaches, these people said.

They also are discussing the possibility of examining who hacked J.P. Morgan and others that may have been affected by the recent breach, these people said. The group also may look at what information was taken, these people added.

A J.P. Morgan spokeswoman said the bank communicated with customers three times to provide details about the cyberattack. She said the firm waited until it had gathered more facts to give detailed disclosure and it wasn't required to notify customers in the first place, given that the breach dealt only with contact information, not sensitive financial information.

Rhode Island Attorney General Peter F. Kilmartin said in a Monday news release that his office sent a letter to J.P. Morgan asking for additional information about the breach and if it would affect Rhode Island consumers. "The scope of this breach is of great concern," he said in the news release.

The office of Connecticut Attorney General George Jepsen has been in contact with the bank regarding the cyberattack since the bank's disclosure earlier this year, a spokeswoman for the attorney general said. She declined to provide further detail, saying it was a pending matter. Illinois Attorney General Lisa Madigan also is looking into the breach. In a statement, Ms. Madigan said that the cyberattack is among the most "troubling" breaches because it shows how vulnerable U.S. institutions and their databases are.

The Wall Street Journal reported Wednesday that investigators believe the hackers that broke into J.P. Morgan targeted at least 12 other financial-services companies, including Fidelity and E*Trade Financial Corp., people familiar with the matter have said. Besides Fidelity and J.P. Morgan, it's unclear if any other firms had information taken.

Fidelity, the mutual-fund shop that made investor Peter Lynch famous, has broadened out its business in recent years. The company now has about 69 million customer accounts, and its website notes that 20,000 businesses use Fidelity to manage employee-benefit programs.

Write to Emily Glazer at emily.glazer@wsj.com

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