The largest power blackout in American history prompted new calls Friday for overhauling the nation's electricity system even as investigators searched for clues to what might have triggered power outages from New England to Michigan.

There were indications the blackout may have been triggered not in upstate New York or Canada, as many have speculated, but somewhere along Lake Erie in Ohio, according to the industry-sponsored group that monitors the transmission system.

"That's where the information is starting to point," Ellen Vancko, a spokeswoman for the North American Electric Reliability Council, said in an interview. "It looks like that's where the collapse started."

Vancko said it would take time to pinpoint the cause.

But Michehl Gent, the NERC president, said he was fairly confident terrorism wasn't involved.

"We don't have any indication of blown-up equipment," he said on ABC's "Good Morning America." "So, we're almost certain it's not terrorism of any kind."

New York Gov. George Pataki said the cascading problem should have been isolated by safeguards in the system. "That just did not happen," he said on NBC's "Today" show.

A member of the federal agency that regulates transmission lines said the resumption of power also was being hampered because the "transmission system — our (power) highway — is so weak and so fragile."

"It's very clear this is not about deregulation. It's about investing in the transmission system," said Nora Brownell, a member of the Federal Energy Regulatory Commission. Some critics of increased competition in the electricity industry said it has caused utilities to concentrate on markets and ignore investing in the transmission grid.

Brownell said it was still unclear what happened.

Was it lightning or fire? Born in America or Canada?

It depends on who's talking.

The early confusion underscored the bedeviling complexity woven into the North American electricity network in recent decades, the boom in cross-border power trading, and the interdependence of the many parts and partners multiplied by energy deregulation.

The blackout already has spawned talk of overhauling the national electrical grid which many characterized as antiquated and raised new questions about whether deregulation of the power industry might have played a part in Thursday's disruptions.

Like a river's tributaries, their contributions spill into immense regional power grids, where they become anonymous and untraceable. Managers at dozens of control sites monitor intricate crosscurrents of supply and demand, watching over their delicate balance.

A single failure can reverberate through the system and set off a catastrophic chain reaction like Thursday's, even with the relatively moderate summer heat and humidity of that day.

Investigators eager to find the cause were focusing on transmission lines and transformers in upstate New York and neighboring Canada, scouting for traces of the surge that tripped circuit-breaking protective devices to shut down the system.

President Bush promised a review of "why the cascade was so significant, why it was able to ripple so significantly throughout our system."

"We're the world's greatest superpower, but we have a Third World electricity grid," said former Energy Secretary Bill Richardson, now governor of New Mexico.

Most accounts of what went wrong late Thursday afternoon agreed on only a few points. Federal and local officials, including Bush, saw no apparent sign of terrorism. There was a bubble of speculation about whether computer hackers might be responsible, but no real evidence so far.

Beyond that, some power network partners — generators, distributors, monitors, governments — dropped early theories blaming one thing or another, but nearly always somewhere else. In truth, no one was initially sure what set off the outages and why they spread so fast and far — across much of the American Northeast, Midwest and southern Canada.

In Canada, the office of Prime Minister Jean Chretien pointed across the border in some early stabs at a cause. It blamed a purported lightning strike, and later a fire, at a power plant in upstate New York. Canada's defense minister, John McCallum, gave another version, blaming a fire at a nuclear plant in Pennsylvania.

Officials on the American side threw back blunt denials and suggested the trouble started to the north. An Associated Press reporter in Niagara, N.Y., reported that the plant there was up and running.

"There's not even a trash can fire. We would know," added Maria Smith, a spokeswoman for the Pennsylvania Emergency Management Agency.

Some American officials, including a spokeswoman for New York Gov. George Pataki, shifted scrutiny to a possible problem in transmission lines between New York and Canada — probably on the northern side, they said.

Canada's defense minister later inched away from some of the first Canadian explanations — but still blamed the Americans for the problem.

Such confusion between energy partners has sometimes been blamed for the outages themselves in the past, including a nine-state Western blackout in 1996 laid partly to weak coordination.

While answers are scant for the moment, investigators of the latest blackout will probably abound. Federal and state agencies, and Congress, are expected to jump in. They will ultimately want to figure out why safety measures failed to shield more grids from the collapse and, of course, who to blame.