It may have started with something as simple as a lightning strike in a remote corner of New York State. Within a few minutes or perhaps even seconds, a cascading series of failures and deliberate shutdowns had cut electric service to tens of millions of people from Toronto to Maryland, and as far west as Detroit.

Exactly how it happened was still murky yesterday as power companies scrambled last night to restart the system, plant by plant.

But at least one thing was abundantly clear: Extensive contingency planning by the region's power suppliers had only partly ameliorated the glaring weaknesses that were exposed by the last two disastrous New York blackouts, in 1977 and 1965.

"This is something that people thought wouldn't happen again, but nothing is perfect, and nothing is failsafe," said Richard Kessel, chairman of the Long Island Power Authority. "Obviously, the entire industry is going to have to take a hard look and determine what happened."

Based on the imprecise information that was available late yesterday, it appears some unexpected event near Niagara Falls -- possibly a lightning strike -- overloaded a major transmission line, instantly sending huge amounts of electricity surging onto adjacent lines. Quickly, large parts of the interconnected system of power plants and lines that make up the Northeastern grid were shut down -- exactly what happened in previous blackouts. The 1977 blackout, for example, was triggered by lightning strikes that hit several lines simultaneously.

"If you hit it too hard in some places, it starts to collapse," said Ian Goodman, president of Goodman Group Ltd., in Berkeley. Calif. "That's why in a few minutes a whole area of the United States can lose power. That's what happened in '65. That's what happened in '77. And that seems to be what happened now."

Other experts suggested it was far too early to try to identify a specific cause, because when the grid shuts down, so do the data systems that could provide an explanation.

"We won't know for days what the real answer is," said Robert McCullough, managing partner of McCullough Research, a Portland, Ore.-based consulting company on bulk power.

No matter why the first domino fell, the system collapsed because the sensitive balance between electricity being produced and that being consumed went badly out of whack.

At 4:05 p.m., more power was being generated than was being drawn because customers had been abruptly knocked off the grid, said John Hurstell, director of gas, oil and wholesale power supply for Entergy, an energy company based in New Orleans.

The system grew more and more out of balance until 4:15, when it started to transition back into equilibrium, which is why the entire eastern United States did not lose power. But it was too late for New York and the Midwest.

"The system worked," Hurstell said. "That's why the whole eastern United States isn't out. It isolated the problem and got the balance back between generation and load. But it should have worked faster to prevent any outages."

What made this outage particularly disastrous is that it began at a part of the grid that carries tremendous amounts of electricity between Canada and New York.

"Those are major, major power lines going in and out of Niagara," said Ira Freilicher, a Long Island attorney who represents the New York Independent System Operator. The ISO is the successor to the New York Power Pool, which was set up in the wake of the 1965 blackout to manage the connections between most of the Northeast's major power suppliers -- and thus prevent another major blackout.

The thinking was that if the interconnected system was put in the hands of a single operator, regions that suffered blackouts could be quickly isolated from the rest of the grid by controllers, confining the outage to a small area.

For reasons that were unknown yesterday, the Niagara outage wasn't isolated quickly. Instead, a different innovation after earlier blackouts -- the deliberate shutdown -- kicked in.

Plants that were suddenly overloaded by the loss of power elsewhere shut themselves down within seconds or minutes. In that regard, the system worked, because previous mega-blackouts had caused immense damage to plants throughout the region, forcing them to remain idle for days or weeks while repairs were attempted.

Matthew Cordaro, a former Long Island Lighting Co. executive who is now director of the Center for Management Analysis at the C.W. Post Campus of Long Island University, said yesterday's blackout "sounds a lot like what happened in '65. It's a cascading outage."

Some experts suggested that yesterday's events showed that power companies, pressed to provide more and more power during the hot summer months, had failed to build enough excess capacity to absorb a major surge in one part of the grid without shutting down entirely.

The government hasn't policed the industry and the industry hasn't done a good job of policing itself to ensure reliable transmission, said Robert Blohm, an electric industry adviser.

"Reliability has been managed on a voluntary, self-managed, peer-pressure basis," he said. As a result, he said, "It only takes one system to have a disturbance or failure to have all its neighbors thrown for a loop."

But Frielicher, the lawyer for the New York Independent Service Operator, said it may be that there's simply no way to adequately plan for a contingency such as a major outage on one of the massive Canadian transmission lines.

"When there's a sudden disruption on the system somewhere, such as a major substation getting taken out, there are cascading effects that happen before anyone can do anything about it," said Freilicher, who, like Cordaro, is a former LILCO executive.

"There may be extra protections that can be built in, and we thought we had already built in extra protections. But even if we had double the capacity, it might not have made a difference in this case because this apparently happened so fast."