A massive power blackout retreated stubbornly Friday as power officials struggled to understand why the historic outage spread in minutes through the northeastern United States and southern Canada.

Lights flickered on and air conditioners restarted for some, but millions of others baked in stuffy rooms.

Much of Pennsylvania was unaffected; the lights remained on in Philadelphia and Pittsburgh.

PJM Interconnection, the Valley Forge, Pa.-based company that manages the power grid in much of the mid-Atlantic region, said it saw a massive outflow of power shortly after 4 p.m., but that its network quickly isolated itself, stopping the blackouts from spreading south.

Investigators are focusing on transmission lines and transformers in upstate New York and neighboring Canada, looking for a surge that tripped circuit-breakers and shut down the system.

Until there's a definitive answer, speculation is running rampant and blame is being tossed back and forth across the U.S. border with Canada, as each country insists it's the other's fault.

William Edwards, president of Niagara Mohawk Power Company, said at a news conference that says officials are trying to determine the cause of the blackout. Edwards also said his operators know how the system went down but still don't know why.

Clem Nadeau, senior vice president for operations of Niagara Mohawk Power Company, said he can't completely rule out that lightning strikes a couple of days ago could have impacted machinery in a way that wouldn't show up until now.

Canadian officials originally blamed a lightning strike on an upper new York State power plant, but Canadian Prime Minister Jean Chretien's office later blamed a fire at a New York power plant while the Canadian defense minister said the fire was at a Pennsylvania nuclear plant.

"That is absolutely not true," said Pennsylvania Emergency Management Agency spokeswoman Maria Smith. "It's bizarre. We have a direct line to each of our five (nuclear) power plants and they are all running at 100 percent ... There's not even a trash can fire; we would know."

"We all should wait until we exactly know what happened. ... This should not be a blame game, not at all," Pennsylvania Gov. Ed Rendell said.

Weather experts quickly discounted the Thursday lightning-strike theory and New York Gov. George Pataki, speaking on NBC's "Today" show, said he believed the problem started in Canada, to the west of his state.

While terrorism was swiftly ruled out by President Bush and other officials, there was scant indication of what had caused the outage, which began on the cusp of Thursday's afternoon rush hour in Eastern cities.

The New York Independent System Operator, which runs the state's wholesale electricity market and monitors power usage, said it had detected a sudden loss of power generation at 4:11 p.m.

Kenneth Klapp, an ISO spokesman, said the problem was detected from information on power usage and transmission prior to, during and after the blackout. The ISO had not determined the exact location of the problem by early Friday.

More generally, industry and government experts blamed a system composed of interconnected grids that has not been upgraded to meet power demands.

Electric industry and government officials said the nation's power grid has needed major upgrades for years, but industry experts said there were three major obstacles in the way: the expense, environmental opposition and people who didn't want power facilities near their back yards.

Both federal and state agencies, as well as congressional committees, are expected to investigate the blackout and try to determine why measures put in place to isolate grids and keep power disruptions from spreading failed to do so.

The blackouts easily surpassed those in the West on Aug. 10, 1996, in terms of people affected. Then, heat, sagging power lines and unusually high demand for electricity caused an outage for 4 million customers in nine states.

An outage in New York City in 1977 left 9 million people without electricity for up to 25 hours. In 1965, about 25 million people across New York state and most of New England lost electricity for a day.