Security experts believe that Iran has declared ‘cyber war’ on the US, claiming that the Islamic nation was almost certainly responsible for a spate of online attacks against American banks last September.

“There is no doubt within the U.S. government that Iran is behind these attacks,” said James A. Lewis of the Center for Strategic and International Studies in Washington in an interview with the New York Times.

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Dozens of America’s largest banks were hit during the Distributed Denial of Service (DDoS) attacks, including Bank of America, Citigroup, HSBC, JP Morgan Chase, Wells Fargo and others. As a result their websites went down or were only accessible intermittently for a number of days before authorities were able to block the malicious traffic. Just last month, it’s believed that the same hackers launched a second wave of attacks, this time targeting five banks and again crippling their websites.

Experts said at the time that the attacks were unprecedented in their size, citing the excessively large amount of bandwidth flooding their websites. Investigators have since discovered that the hackers pulled off an audacious trick to generate that enormous flow of traffic, infecting dozens of data centers around the world with a virus called Itsoknoproblembro and then using these to direct the attacks.

According to the NYT, it was precisely due to this that the finger of suspicion pointed at Iran. The huge amount of traffic, coupled with the level of know-how required to pull of such a stunt would almost certainly require government sponsorship of some sort, and Iran has more reason than most to engage in such an attack. Another reason US officials are so convinced that a state was behind the attacks is the fact that the hackers made no attempt to steal private data or customer information.

Establishing a motive is simple enough – the Iranians see themselves as victims having had seen their own nuclear facilities come under attack from sophisticated viruses such as the Stuxent malware, which is believed to have been planted by the US or Israel to stall progress on its suspected nuclear weapons projects.

At the time of the attacks, a little known group calling itself the Cyber Fighters of Izz ad-din Al qassam claimed responsibility, adding that they were protesting at the controversial ‘Mohammed mocking’ video posted on YouTube. However, officials are now convinced that this was either a smokescreen or simply just a fraudulent claim.