America's worst blackout has been blamed on Ohio's FirstEnergy firm after a three-month investigation.
The investigators said the company's operators were poorly trained and computer problems kept them from recognising the scale of the problem immediately. The investigation also criticised Midwest regional monitors over the power cuts in August.
US Energy Secretary Mr Spencer Abraham said: "This blackout was largely preventable." The blackout left 50 million people in eight US states and eastern Canada in darkness.
The report said Midwest grid monitors should have been able to observe the failure of three FirstEnergy, high-voltage lines in northern Ohio.
The monitors also should have helped the utility company respond to the problem and head off the cascade of outages that engulfed a region from eastern Michigan to New York city in seven minutes.
The report found that FirstEnergy, the nation's fourth-largest utility holding company with 4.3 million customers, had violated four industry grid reliability standards. It also said that MISO, the regional grid monitor, had broken several rules in connection with the blackout.
The industry largely regulates itself when it comes to grid reliability with no penalty for rule violations.
AP