or several years, as growing summertime surges of power criss-crossed the web of transmission lines in Ohio and the rest of the Midwest on their way to Manhattan or Toronto or Detroit, utility and government monitors grew increasingly concerned about the risks of straining the system.

Problems on the lines were becoming more frequent, and a series of reports, by the industry's own quality-control offices and government agencies, described the risks and urged utilities to be more aware of the physical limits of the Midwest system.

The complexity and magnitude of the power flows, one report by the Federal Energy Regulatory Commission said, could "overwhelm the electronic and software tools used to model and manage power flows on the grid."

So when an hourlong series of line failures and plant shutdowns suddenly exploded on Thursday into North America's biggest blackout, few experts were surprised at where it had begun.

"Reliability here," said Dr. Stephen A. Sebo, an electrical engineering professor at Ohio State University and a member of the Ohio Power Siting Board, "is unquestionably a sore issue."

Now it is the focus of an international inquiry. And Midwestern and Ohio power managers and the FirstEnergy Corporation, an Akron utility that struggled for an hour before the full catastrophe to deal with the failure of several of its lines, find themselves enduring intensifying scrutiny and criticism.

Among the questions officials want to answer is whether anything was done by Midwestern or FirstEnergy officials to alert neighboring regions to the growing crisis. They may also want to determine whether enough was done over the years to manage the Midwestern system responsibly. Finally, the fact that the blackout occurred on a day of only ordinary summer demand suggests the problem lay with the way the system was managed and not with a lack of capacity, some critics said.

At FirstEnergy, Todd Schneider, a spokesman, said yesterday that the company was far from ready to say that the precipitating event was the failure of four of its lines and one co-owned by American Electric Power. Three hours earlier, he said, there were signs of instability on the grid outside FirstEnergy's service territory. He would not say where they occurred, but said they could have played a role.

"There were situations throughout the region that may have contributed to the event, not just these power lines in our system," he said.

FirstEnergy distributed a statement on Saturday saying that it had failed to notice the line failures because an alarm system designed to alert its operators was not working.

Yesterday, Mary Lynn Webster, a spokeswoman for the Midwest Independent Transmission System Operator, the industry group that monitors the flow of power throughout the region, said that during the escalating problems in Ohio, at least three conversations took place between coordinators from that group and FirstEnergy to discuss the line failures. She said tapes of the conversations would be reviewed.

Power managers in adjacent parts of the grid, including the Northeast, said they were concerned that there was no early warning, adding that such an alert might have helped them gird for what one official called the eastward-pulsing "shock wave" that triggered the wider failure.

Last night, leaders of an industry investigation of the blackout said it was too soon to assign blame.

"What we really need to focus on now is why protective schemes that should have localized the impact of various events did not work," said Michehl R. Gent, the president of the North American Electric Reliability Council, a nonprofit industry group set up after the blackout of 1965 to cut the risks of such failures.

Its inquiry is just one of a host of investigations trying to unravel how a building parade of problems around Ohio was not isolated before the problem crossed the maze of lines linking the Midwest and the East, which collectively are called the Eastern Interconnection.

There, within nine determinative seconds, the problem exploded into a power failure affecting 9,300 square miles and millions of people.

The investigators range from teams dispatched by dozens of individual companies inspecting gear and records for strained or failed equipment to an international task force created by the top energy officials in the United States and Canada. They will need to disentangle what Mr. Gent described as more than 10,000 "discrete system events," scattered over thousands of miles of interconnected systems as big as a billion-watt power plant and as small as the diameter of a high-tension power line.