The report by a U.S.-Canadian task force said the FirstEnergy operators did not respond properly, allowing the Aug. 14 outage to cascade, eventually cutting off electricity to 50 million people in eight states and Canada.
The task force also cited outdated procedures and shortcomings at a regional grid monitoring center in Indiana that kept officials there from grasping the emerging danger and helping FirstEnergy deal with it.
"This blackout was largely preventable," Energy Secretary Spencer Abraham said in remarks prepared for a news conference.
Among the faults found at FirstEnergy was a simple failure to keep trees around power lines trimmed.
The report had been expected to put the blame for the Aug. 14 blackout, which knocked out electricity for 50 million Americans and Canadians, on a series of missteps rather than one single event.
Abraham said earlier this month that the task force found no one cause for the power outage that cascaded from Ohio and Michigan into Canada and down New York State to the East Coast.
In a report Tuesday to the Senate Committee on Government Affairs, the General Accounting Office pointed to a series of transmission line and large power plant failures in mid-Ohio, Michigan and the Cleveland area in the two to three hours before the outage.
A power grid expert says the regional system should remain loosely connected.
"You tie them all together strongly and you're going to send the problem cascading throughout the whole system," Enver Masud, who once managed the U.S. National Power Grid Study for the U.S. Energy Department, told CBS News Correspondent Peter Maer.
The Michigan Public Service Commission, in a report earlier this month, criticized a lack of coordination among two Ohio utilities that were experiencing major power line failures — FirstEnergy Corp. and American Electric Power — and two regional industry entities that direct power line flows in the upper Midwest and mid-Atlantic states.
The commission said representatives of all four organizations were conferring with one another about the unfolding events but that no one of them was able to put all the pieces together to see the big picture.