Wells Fargo is the most recent mega-bank to be hit by a distributed denial-of-service attack. According to the Wall Street Journal, roughly 220 customers filed complaints of outages on its Web site today saying they had problems logging on.

"The amount of bandwidth that is flooding the websites is very large, much larger than in other attacks, and in a sense unprecedented," chief executive of private security firm CrowdStrike Dmitri Alperovitch told the Wall Street Journal.

Last week, similar attacks happened on J.P. Morgan Chase and Bank of America's Web sites. Users would try to log-on and instead got outage messages from the banks. According to the Wall Street Journal, there were 330 complaints about Bank of America's disruption and nearly 1,000 complaints about J.P. Morgan's.

The big question here is who is launching these attacks?

An unknown group called the "cyber fighters of Izz ad-din Al qassam" is claiming responsibility. Over the past week, it has been posting details on Pastebin about its attacks on U.S. banks, saying that it's retaliating for the release of the controversial video posted to YouTube that mocks the Prophet Mohammad. The film has sparked furor and demonstrations across the Middle East and led to the death of the U.S. ambassador in Libya and three others.

"In the previous announcements we stated that we will not tolerate insulting exalted character of the prophet of mercy and kindness," Izz ad-din Al qassam wrote in a new Pastebin post today. "Due to the insult, we planned and accomplished a series of cyber operations against the insulting country's credit and financial centers."

Izz ad-din Al qassam wrote that the attacks will continue until the video is removed from "the Internet." The group said it will attack different banks for eight hours a day, every day. In a schedule of its attacks, the group wrote that on September 25 it will strike Wells Fargo, on September 26 it will go after U.S. Bank, and on September 27 it will take down PNC Bank.

It is not clear if Izz ad-din Al qassam is indeed the cause of all of the bank's outages.

Wells Fargo rarely experiences Web site problems, according to the Wall Street Journal, so the fact that it was down today is significant. A spokesperson for the bank confirmed with the Journal that customers were having difficulty logging on but that customer-account information was safe since the outage was due to systems issues rather than account issues.

CNET contacted Wells Fargo for more information and we'll update the story when we hear back.