Security researchers said the attack methods were too basic to have taken so many American bank sites offline. The hackers appeared to be enlisting volunteers for the attacks with messages on various sites. On one blog, they called on people to visit two Web addresses that would cause their computers to flood banks with hundreds of data requests a second. They asked volunteers to attack banks according to a timetable: Wells Fargo on Tuesday, U.S. Bancorp on Wednesday and PNC on Thursday.

But experts said it seemed implausible that this method would create an attack of this scale. “The number of users you need to break those targets is very high,” said Jaime Blasco, a security researcher at AlienVault who has been investigating the attacks. “They must have had help from other sources.”

Those sources, Mr. Blasco said, would have to be a group with money, like a nation, or botnets — networks of infected computers that do the bidding of criminals. Botnets can be rented through black market schemes that are common in the Internet underground, or lent out by criminals or governments.

Last week, Senator Joseph I. Lieberman of Connecticut, chairman of the Senate Homeland Security Committee, said in an interview on C-Span that he believed Iran’s government had sponsored the attacks in retaliation for Western economic sanctions. The hacker group rejected that claim. In an online post, it said the attacks had not been sponsored by a country and that its members “strongly reject the American officials’ insidious attempts to deceive public opinion.”

The hackers maintained that they were retaliating for the online video. “Insult to the prophet is not acceptable, especially when it is the last Prophet Muhammad,” they wrote.

It is very difficult to trace such attacks back to a particular country, security experts say, because they can be routed through different Internet addresses to mask their true origin.