Iranians have mounted a series of denial-of-service attacks over the past year that target major US banks and other companies, according to two published reports that cite unnamed US officials.

The reports, published on Friday by The Washington Post and Reuters, came a few days after websites for both Bank of America and JPMorgan Chase experienced unexplained service disruptions. US Senator Joseph Lieberman, chairman of the Senate Homeland Security Committee, said on Friday that he believes a unit of Iran's Revolutionary Guard Corps is behind the disruptions, but provided no evidence to support the claim. Neither bank has confirmed that the disruptions were the result of attacks, so it's possible equipment failure or other internal causes are responsible.

According to the Washington Post, US officials suspect that Iran was behind similar denial-of-service attacks, which bring websites to a crawl or make them completely unavailable by overwhelming them with garbage traffic. One such attack was carried out in August, and was aimed at disrupting the websites of oil companies in the Middle East "by routing their efforts through major US telecommunications companies, including AT&T and Level 3," the publication reported, citing US intelligence and industry officials. It was the largest attempted DoS attack against AT&T "by an order of magnitude," an industry official said. The sources spoke on condition of anonymity because they weren't authorized to speak to the press.

According to Reuters, Citigroup has also been targeted in the campaigns, which it said are likely in retaliation for their enforcement of Western economic sanctions against Iran. Reuters also said while the attacks originated in Iran "it is not clear if they were launched by the state, groups working on behalf of the government, or 'patriotic' citizens." The attacks may be intended to distract victims from other, more destructive breaches, the news organization added.

Security experts have long said that it's difficult or impossible to determine the origin or source of many DoS and other computer-based attacks. In the absence of technical evidence that supports claims attacks are coming from Iran, it's not possible to verify them.