hree months before the biggest blackout in North American history, the agency charged with protecting the nation's electrical grid put industry officials on notice that the section of the grid covering Ohio and other parts of the Midwest was particularly vulnerable to the kind of "cascading events" that unfolded last Thursday.

Officials with the group, the North American Electric Reliability Council, in a report that singled out the Midwest as the only part of the country at risk of such a devastating event, said the region could face "large, unanticipated power flows" this summer and would have to be prepared to handle the challenges with care.

Officials at the reliability council, an industry group, lacked the authority to order the utilities to take additional steps to prevent a small local failure from snowballing into a catastrophic one. And they concede that they did not even informally suggest anything more be done to protect that section of the nation's fragile grid. But the report makes it clear that the industry had been notified of the threat in the region and the fine margin for error that existed.

That margin for error, many experts now say they believe, disappeared last Thursday afternoon after several power lines went down in Ohio. That was the beginning of a blackout that ultimately extended from Detroit to Toronto to New York, they say. While investigators say they have weeks of work to do before understanding the precise causes and chronology of the electrical collapse, the failures of Midwestern transmission lines and power plants and the warning system meant to limit the spread of trouble are considered pivotal.

Officials at the FirstEnergy Corporation, the company responsible for the lines that started failing before any widespread blackout, declined yesterday to comment on the report and to say whether the company had taken any steps in response to the warning.

Energy officials and other experts say the existence of the report makes it all the more critical for investigators to determine whether the region's utilities, having received such an explicit reminder about their obligations, followed the proper protocols and met those challenges on that day.

Officials with the reliability council confirmed that the mention of potential cascading power failures in the region was a significant warning. "Our reliability assessments don't mention problems lightly," said Ellen Vancko, a spokeswoman for the group, created after the 1965 blackout to prevent such calamities.

Since the deregulation of the energy industry, the section of the Midwest grid identified in the report has become one of the great crossroads in the transmission of power across the nation — a kind of Times Square in the flow of electrical traffic. Power produced as far away as Denver flows through the Midwestern grid on its way to users in New York and elsewhere.

The assessment by the reliability council, issued in May, would have been seen by virtually all controllers of the grid nationwide, said Mary Lynn Webster, a spokeswoman for the Midwest Independent System Operator.

"Anyone in grid management in the United States would be looking at it," Ms. Webster said.

The Midwest Independent System Operator is charged with helping oversee the safe generation and transmission of power in the Ohio area, although responsibility for taking action in a crisis rests with individual utilities.

Ms. Webster said that as the report suggested, this part of the grid was susceptible to "unanticipated flows because of something that is happening somewhere else on the system," like commercial transactions between companies outside the region that nevertheless pour electricity through its boundaries.

Those conditions can also be caused by an unexpected power failure or a generating unit that breaks down on the local grid, she said. But she cautioned against assuming that this was the only part of the national grid that was prone to those conditions. "There are areas in all parts of the country that are constrained," Ms. Webster said.