Bank of America Tower. Photo: Jleon.

A wave of online attacks has targeted several dozen online banking websites in a coordinated attack that U.S. government officials believe was perpetrated by Iran, the New York Times reported.

“There is no doubt within the U.S. government that Iran is behind these attacks,” said James A. Lewis, a former official in the State and Commerce Departments and a computer security expert at the Center for Strategic and International Studies in Washington, DC.

Several prominent U.S. banking institutions such as Bank of America, Citigroup, Wells Fargo, U.S. Bancorp, PNC, Capital One, Fifth Third Bank, BB&T and HSBC, have been targeted since September.

The hackers have used advanced encrypted denial of service attacks (DDoS), which cause large traffic spikes to sites until they collapse.

A hacker group calling itself Izz ad-Din al-Qassam Cyber Fighters is claiming responsibility for the attacks. U.S. intelligence officials believe that the group is a cover for Iran, which may be seeking revenge against U.S.-led international sanctions and previous computer viruses such as Stuxnet, which have disrupted Iran’s nuclear progress.

Computer security experts believe that the level of sophistication behind these attacks suggests the state backing and not amateur hackers. Additionally, the attacks did not attempt to steal money, another hallmark of amateur hackers.

“The scale, the scope and the effectiveness of these attacks have been unprecedented,” said Carl Herberger, a computer security expert.  “There have never been this many financial institutions under this much duress.”