"The lights are on," declared New York Gov. George E. Pataki (R).

But Pataki and other officials also cautioned residents to conserve as much energy as possible to avoid overloading the delicate electrical grid as engineers worked to bring more coal- and nuclear-powered plants back on line by Sunday. Energy Secretary Spencer Abraham (news - web sites), appearing at a news conference with Pataki, warned that "there remains the potential for rolling blackouts in some areas."

Ontario Premier Ernie Eves said that "Monday morning will be the big test. There is no way we will be, quote, normal on Monday morning. People can't be frivolous with their use of energy."

Investigators said today that the cascading blackout, which toppled numerous power systems within seconds of 4:11 p.m. Thursday afternoon, was likely caused when power suddenly stopped flowing through three transmission lines in Ohio, including one that may have come in contact with a tree. FirstEnergy Corp., which owns those lines, put out a written statement later saying that a unit in one of its power plants shut down before the lines failed. But officials and energy experts were still at a loss to explain how an isolated problem could have led to Thursday's catastrophic power loss across the Northeast and Canada.

Officials in the United States and Canada said a joint task force, created late Friday by President Bush (news - web sites) and Canadian Prime Minister Jean Chretien, will also try to determine a cause for the power outage and a plan for preventing it from happening again.

The extended period without electricity also left lingering health concerns. In Cleveland and Detroit, where the blackout had crippled water-pumping stations, officials urged residents to continue to boil their drinking water. In New York, sanitation workers struggled to clear mountains of leftover garbage, while health authorities warned tourists about spoiled food and enforced a ban on swimming at city beaches affected by overflowing sewage.

Detroit Mayor Kwame M. Kilpatrick announced that to discourage the use of cars, public transportation would be free through the end of the weekend. And he said that automakers and other industries would remain closed until Monday to help minimize the use of electricity.

Airlines at New York's three major airports and those in Cleveland and Detroit were still struggling to resume normal service. Many of New York's major museums remained closed.

But officials and residents in the largest cities affected by the blackout spent much of the time today savoring the calm streets that marked the two-day crisis. In Detroit and New York, both well known for looting and civil unrest during previous crises , police reported fewer felonies and street crimes than normal for mid-August. Both cities had saturated the streets with police officers

Across the affected area of the United States, only a handful of fatalities was recorded, including a 6-year-old boy and a 40-year-old man who died in separate fires. A 42-year-old woman in Connecticut died in a blaze sparked by a candle.

Overnight Friday in Detroit, before power was fully restored, only six people were arrested for incidents related to the blackout, according to city officials.

"There were no major incidents, no major spikes in crime," Kilpatrick said at a news conference today. "Everyone was bracing for the worst. It was never going to happen. It did not happen."

One of the few businesses apparently targeted by looters was a Citgo station on Detroit's west side, where windows were shattered and the contents of shelves strewn across the floors. Yet DeLisa Davis, standing across the street from the ravaged gas station this morning, said she had feared much worse.

"I really thought it would be something like a riot," Davis said. "Especially in this neighborhood. But people really kept it under control. It could have been so much worse."

In Cleveland, police logged 19 arrests on Thursday night -- less than half the average of 50 in the summer, according to Mayor Jane L. Campbell. The city deployed twice the usual number of officers.

"Because of 9/11, people are calm now," said Cleveland resident Tom Westerfield, 30, who works at Tower City mall downtown, where shoppers and tourists congregated today in numbers that mall employees said approached normal. "I think as a society, we're more mature. This just isn't that big a deal."

Electricity and water service were fully restored in Cleveland, although city officials still asked the city's 1.5 million water customers to boil water to kill any possible contaminants. There were pockets without water this afternoon, but officials said those were the result of eight water main breaks unrelated to the power outage.

In New York, residents stocked up empty refrigerators, shared blackout stories and cheered the return of badly needed air conditioning. Subways rumbled beneath the streets again. Broadway had reopened late Friday after a night of canceled performances, and tourists today lined up to get into the theaters. Services returned largely to normal, though some still complained they had no hot water.

The exceptions were restaurant, grocery store and bodega owners, many of whom spent today throwing away the rest of their spoiled food and totaling what the blackout cost them. City officials estimate the first day cost businesses $750 million, and took $50 million in tax revenue from city coffers.

At Fairway, a popular market on the Upper West Side, store manager Steven Jenkins, 52, had the rumpled look of a man who had gotten little sleep over the last 72 hours.

"We tossed about seven Dumpster loads of stuff," he said. "Chicken, beef, milk, eggs. Prepared foods. I have no idea how much it all costs. Tens of thousands of dollars. It's been just a total nightmare. The porters can't get in to work, so the store is filthy."

After telling one customer five times how to find the dried red peppers, Jenkins said his patience was at an end. "This place has just been a madhouse today."

In Union Square, it was brunch as usual at Coffee Shop, as diners talked about how New York conquered the blackout as it conquers everything else. Many also had high praise for Mayor Michael R. Bloomberg (R), whose reassuring demeanor prompted comparisons to former mayor Rudolph W. Giuliani after the Sept. 11 attacks.

"I thought the city did amazingly well," said Karen Hynes, a 31-year-old software company manager. "The cops were great. I thought Mayor Bloomberg was great, too, very matter-of-fact and calming. I don't think Giuliani could have done it any better."

Shoppers and tourists poured back into the subways, but for some the return of service was a mixed blessing. Riding the Q train in from Queens, Cathy Rodriguez, 22, said she wished it had taken a day longer for service to be restored.

She had spent Thursday night on the floor of her Herald Square law office only to be forced out of the building, which had no air conditioning, by Friday's stifling heat. "I don't know how happy I am about the train running again," she said. "I'm just going back to work."

Snyder reported from Cleveland. Also contributing were staff writers Ben White and Juliet Eilperin in New York, and Robert Pierre and Jonathan Finer in Detroit.