The FBI is investigating intrusions into the computers of banking giant JPMorgan Chase and several other U.S. banks, officials said Wednesday night.

“We are working with the United States Secret Service to determine the scope of recently reported cyber attacks against several American financial institutions,” FBI Supervisory Special Agent Joshua Campbell said in a statement.

The FBI did not comment on who was thought to be behind the intrusions, first reported by Bloomberg News.

The penetration at JPMorgan resulted in the exfiltration of gigabytes of sensitive data, and Bloomberg reported that it was the work of Russian hackers. But one industry official said it was unclear who conducted the attacks.

Bloomberg said the FBI was investigating whether it was in retaliation for Western sanctions levied on Russia because of the situation in Ukraine.

But some analysts discounted the theory, noting that retaliation involves destruction or disruption of networks — not theft of data.

“We have seen Russian intelligence services target financial institutions for the purpose of espionage,” said Dmitri Alperovitch, co-founder of CrowdStrike, a cybersecurity firm, who was not commenting on the JPMorgan case.

Of particular interest to them is data from the oil and gas trading desks, Alperovitch said. Oil and gas are major sources of revenue for Russia.

One industry official said the intrusion was discovered in the middle of this month. A second industry official confirmed the investigation, but said there was no indication of increased fraud activity.

“Companies of our size unfortunately experience cyberattacks nearly every day,” said Trish Wexler, a spokeswoman for the bank. “We have multiple layers of defense to counteract any threats and constantly monitor fraud levels.”