The cyber attackers gumming up bank websites shifted to Pittsburgh's PNC Bank on Thursday, with U.S. Bancorp saying that its public website was back to normal.

U.S. Bank's website was hit Wednesday by disruptions from a "denial of service" attack that bombards websites with traffic. It caused pages to load slowly and prevented some customers from accessing the site and their accounts.

On Thursday, a spokeswoman for the Minneapolis-based bank said its systems "are performing well."

"We are continuing to monitor the situation, and we will take any precautions we deem necessary," spokeswoman Nicole Garrison-Sprenger said. "We are also continuing to work closely with federal law enforcement officials to address the issue."

The technical attacks disrupting websites have hit JPMorgan Chase & Co., Bank of America Corp. and Citigroup Inc. in the past year, according to a report from Reuters, which cited unnamed sources. The denial-of-service attacks flared up last week, and on Tuesday spread to Wells Fargo & Co. and on Wednesday to U.S. Bank.

KeyCorp's Key Bank also has been said to be affected. On Thursday, a spokesman for the Cleveland-based lender wouldn't confirm or deny any attacks but said "we have experienced significant system traffic over the past several days." The systems are "safe and secure," he said.

It's not clear who is behind the campaign.

Sen. Joe Lieberman, I-Conn., chairman of the Senate Homeland Security & Governmental Affairs Committee, has said he thinks the campaign is being coordinated by Iran in retaliation for sanctions on Iranian financial institutions.

But an Islamic group calling itself Izz ad-Din al-Qassam Cyber Fighters has posted notices on the Internet saying it is leading the campaign in retaliation for the video on YouTube ridiculing the prophet Mohammed, saying the attacks will continue until the anti-Islam video is erased.

Fred Solomon, a spokesman for PNC Financial Services Group Inc.'s PNC Bank, said the high traffic situation Thursday was very similar to what other banks have experienced.

"Some customers got right through, some customers had to try several times," Solomon said.

Jennifer Bjorhus • 612-673-4683