The Associated Press
Iran's state TV says the hackers were Sunni Muslims based outside Shiite Iran, suggesting a sectarian motive, though it did not say how it knew that.
The semiofficial Fars news agency said the hackers were based in the United Arab Emirates and blocked access to 300 sites. The Emirates is home to a large Iranian community, some of whom oppose Iran's clerical rule.
Thursday's Fars report called it the most serious cyber attack of its kind in the past few years.