By Emily Glazer 

J.P. Morgan Chase & Co. said Thursday morning it isn't seeing "unusual fraud" and it is working closely with law enforcement to determine the scope of a computer-hacking attack.

The comments from J.P. Morgan come a day after the Federal Bureau of Investigation said it said it was "working with the United States Secret Service to determine the scope of recently reported cyber attacks against several American financial institutions."

The investigation is focused on J.P. Morgan and as many as four other banks, in what people familiar with the probe described as a significant breach of corporate computer security.

On Thursday, the largest U.S. bank by assets added that it is taking "additional steps" to safeguard sensitive or confidential information and will contact relevant parties as it learns more about who may have been impacted. It stressed customers should contact the bank -- as always -- if any suspicious activity on their accounts is detected and they will not be liable for fraud

Cybersecurity experts began probing the possible J.P. Morgan breach earlier this month, according to people familiar with the investigation.

Those people said the evidence gathered so far suggested hackers were able to make a significant foray into J.P. Morgan's computer system. People with knowledge of the probe said it appeared between two and five U.S. financial institutions may have been affected. The names of all targeted banks couldn't be immediately determined.

The attack appears to have been caused by malicious computer code, known as malware, according to a person familiar with the matter.

Thefts of U.S. corporate data have in the past often come from hackers based in China, Russia or the former Soviet Union, though that doesn't mean the cyberattacks involve those governments. Just as in the U.S., hackers in those countries can act on their own and sell stolen data to other organizations.

The style of the attacks and the targets--large U.S. financial institutions--have led some people briefed on the investigation to suspect a possible Russian or Eastern European link. Russian organized crime often targets large financial institutions. But several people with knowledge of the investigation cautioned it is too early to tell who was behind the attacks.

Hackers appear to have originally breached J.P. Morgan's network via an employee's personal computer, a person close to the investigation said. From there, the intruders were able to move further into the bank's inner systems. Employees often use software to tap in to corporate networks from home through what are known as virtual private networks.

Such an attack would mark the latest instance in which a large corporate network was breached by a weak external link. When hackers stole 40 million payment-card numbers from Target Corp. last year, they originally infiltrated the retailer by stealing a ventilation contractor's password.

In mid-August, cybercriminals hacked in to nearly 1,000 grocery stores around the U.S. The common link: Supervalu Inc. of Eden Prairie, Minn., which managed the stores' technology services and had remote access to those locations, people familiar with that incident have said.

In recent weeks, J.P. Morgan called numerous security vendors with concerns it had a problem, people close to the investigation said. The bank in recent months hired a number of employees with Defense Department experience because the firm treats cybersecurity as a problem akin to military security, people familiar with the matter said.

Cybersecurity has been a chief concern--and cost--for large banks over the past few years.

J.P. Morgan, along with other banks, has been vulnerable to attacks in the past, particularly so-called distributed denial of service threats, known as DDoS. These attacks knock websites offline by flooding them with useless traffic. Iranian hackers aimed a DDoS attack at J.P. Morgan, U.S. Bancorp, PNC Financial Services Corp. and Wells Fargo & Co. in 2012, according to U.S. officials.

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

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