hortly after 4 p.m. on Thursday, Aug. 14, the power grid in New York State was operating with textbook efficiency, briskly exporting 1,000 megawatts of electricity, roughly the output of a large power plant, northward across the border to Ontario. Several power plants were out of service in Canada, and New York was helping answer the need.

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Suddenly, the demand in Ontario nearly doubled, as parts of the power grid to its south, in Michigan and Ohio, began to crumble. Power then began streaming from New York to satisfy the unexpected hunger for electricity on the other side of the border. And then the unthinkable happened: the power flow abruptly reversed, and 1,500 megawatts blew southward in a kind of electrical hurricane on the grid.

It all took 90 seconds, and turned what would have been a major summer blackout in the Midwest into an epic mess that affected tens of millions of people in the largest blackout in the nation's history.

As the investigation into the blackout continues, scientists and engineers will be looking not just at where problems began in the Midwest, where the failure of a number of power lines in Ohio appears to have set everything in motion.

A central question to be answered is what exactly happened at the border of New York and Canada — whether mechanical protective devices at the extremely powerful electrical connection linking the two countries failed or even had a chance to operate, and whether the humans in charge could have done anything to anticipate or limit the damage.

This picture of the blackout's progression is based on information made available by the New York Independent System Operator, which manages the day-to-day flow of electricity on the grid in the state, and industry officials who have been briefed by investigators.

New York officials say that they received no warning of the approaching storm in time to make any manual adjustments to the grid here that could have isolated the state and kept the blackness from speeding southward.

"It's almost like a wave crashing over a seawall," said an industry representative who is familiar with the operations of the northeastern power grid. "It was quick, real quick."

The stunning swiftness of the events defeated the state's best-laid plans for handling such a crisis and keeping the lights on for those millions of people. The grid collapsed here even though New York — unlike other states, like California — has enough power-generating capacity to satisfy its own demands without any outside help.

To explain how the blackout marched through the East Coast from Ontario, investigators will look at everything from the settings on relays — circuit breakers, in essence — on transmission lines connecting the two countries, to potential breakdowns in communications between different pieces of the power grid.

Already, the question of New York's fate — the state was supposed to be safe from a cascading blackout — has become deeply politicized. Democratic lawmakers as well as the state's attorney general have cast blame on Gov. George E. Pataki for a failure of vision and investment in the state's power system. Mr. Pataki has dismissed the criticism and urged that the federal Department of Energy complete its investigation before blame is assigned.

Hearings on New York's performance on Aug. 14 are scheduled for Wednesday in Albany.

Whatever the ultimate sequence of events, the last stage of the blackout was doubly surprising to many officials, given New York's ability to produce enough electricity to isolate itself from the grid and yet still stay powered.

In fact, New York should be able to isolate itself and act independently, said Gavin Donohue, executive director of Independent Power Producers of New York, the association of all the power plant owners. New York can generate more than 34,000 megawatts; peak demand is around 32,000, and that day, they were running at 28,500.