A federal task force of U.S. and Canadian officials will investigate the power blackout and determine how to keep it from happening again.
But even as electricity was returning to much of the affected region, there was still no clear sense of what triggered the breakdown that left millions of people without power and sweltering in summer heat from southern New England to Michigan (search).
The preliminary investigation focused on an electrical transmission loop that encircles Lake Erie. No one was sure where the blackout was triggered, although investigators have been intrigued by a series of interruptions on five power lines in the Cleveland (search) area during the hour before the massive blackout began.
Two minutes after the last of the Cleveland-area line problems there were "power swings noted in Canada and the eastern U.S.," said a document made public late Friday by the North America Electric Reliability Council (search).
But the document cautioned, "It's not clear if these events caused the (wider blackout) or were a consequence of other events."
The White House, meanwhile, announced that the United States and Canada agreed to form a joint task force to identify the cause of the blackout and correct whatever shortcomings caused it. The investigation will be headed by Energy Secretary Spencer Abraham and Canadian Natural Resources Minister Herb Dhaliwal.
"We will find out what caused the blackout and we'll deal with it," President Bush told reporters Friday during a trip to California, a state that suffered its own power crisis two years ago.
Congressional hearings also are planned in September. Federal energy regulators — as well as the industry-sponsored North American Electric Reliability Council (search), or NERC — were also looking into the reasons behind the power grid breakdown.
Power grid monitors expressed shock Friday at how rapidly the disruption of power spread across such a wide region when the grid system was supposed to include safety devices to contain any sudden catastrophic interruption in the smooth flow of power.
"We never anticipated we could have a cascading outage" of this magnitude and speed, said Michehl Gent, chief of NERC, the organization charged with assessing the dependability of the nation's electric grids.
If the problem began in Ohio or Michigan, as speculated, it should never have reached Manhattan, complained New York Gov. George Pataki, adding that the grid was supposed to be designed to isolate such problems. "That just did not happen," he said.
But it may be days, even weeks, before solid answers emerge, said Gent.
Gent at a news conference acknowledged that the answer appeared to be somewhere on what is called the "Lake Erie Loop" — a massive but troublesome transmission system that encircles Lake Erie from New York to Detroit, into Canada and back to New York.
About the time power was disrupted, technicians noticed a stunning development on the northern leg of the loop: some 300 megawatts of electricity moving east abruptly reversed course and within seconds 500 megawatts of power suddenly were moving west.
Electricity flows on its easiest path, so it is believed the change in direction was caused by a sudden reduction in power somewhere on the line at the western end of the loop, investigators suggested.
"This was a big swing back and forth," said Gent, adding that throughout the grid system, power levels began to fluctuate. That caused generators and other systems to trip across the region to protect equipment.
More than 100 power plants, including 22 nuclear reactors in the United States and in Canada, shut down, most of them automatically to protect themselves against power surges, officials said.
But what triggered the shift of electricity flow, and where?
As of late Friday, no one was confident enough to say.
Reports of lightning hitting a facility in the Niagara Falls area have been ruled out, as have reports that a fire at a New York City electric facility may have triggered the power disaster.
The weather also has been given a reprieve because it was not hot enough either in the Ohio Valley or in the Northeast to cause such a demand on electricity that the system should have overloaded, said Gent.
And terrorism has been ruled out by everyone from grid managers to Bush.
But Gent said he wouldn't rule out that negligence by someone, somewhere might have been a cause. Investigators will have to determine whether some industry transmission standards might have been ignored, or perhaps simply conclude that the industry-crafted standards are inadequate.