About 220 customers of the nation's fourth-biggest bank by assets filed complaints Tuesday with sitedown.com, a website that tracks outages.
The complaints detailed problems the customers faced as they tried to log onto Wells Fargo websites.
Sitedown.com received 330 complaints about the Bank of America outage and nearly 1,000 complaints about J.P. Morgan's problems over the past week.
Jeff Herdell, who runs sitedown.com, said the Wells Fargo complaints were significant because the company rarely experiences problems with its websites.
A Wells Fargo spokeswoman confirmed that some customers were having problems logging in on Tuesday but said the San Francisco-based company was working to resolve the issue. She added that the outage was a systems problem and that no customer-account information was compromised.
The scope of the disruption has raised eyebrows.
"The amount of bandwidth that is flooding the websites is very large, much larger than in other attacks, and in a sense unprecedented," said Dmitri Alperovitch, chief executive of CrowdStrike, a private security firm investigating the attacks.
A puzzle for investigators is who is behind the attacks. A group called the Izz ad-Din al-Qassam Cyber Fighters has claimed responsibility for the attacks in online posts as retaliation for what it calls an anti-Muslim film.
"We are aware of the threat and are working closely with law-enforcement authorities," a U.S. Bancorp spokesman said. A spokesman for PNC declined to comment.
Mr. Alperovitch said his firm had found links to prior attacks against other companies that suggest the prior attacks were conducted by the same group. "I don't believe these attacks had anything to do with the anti-Muslim video," Mr. Alperovitch said. "I think the video is an excuse."