NEW YORK (CNN) -- Power slowly flickered on across the northeastern United States and parts of southern Canada after the mysterious massive outage cut electricity Thursday afternoon from New York north to Toronto and west to Detroit -- an area home to some 50 million people.
Times Square, for many the glittering symbol of New York under normal circumstances, sprang back to life about 7:45 a.m. EDT.
The North American Electric Reliability Council (NERC), which monitors the power system, said that an approximate 41,100 megawatts of generated electricity, out of 61,800 megawatts lost, had been restored by 5 a.m. EDT.
With most of his state back on line -- Long Island and parts of five New York City boroughs the exceptions -- New York Gov. George Pataki said that a power failure of this magnitude "shouldn't have happened."
"We have to have answers to this," he said. "We are an energy-dependent society."
Electric regulatory agencies have some "tough questions" to answer about where, how and why a cascading blackout shut down parts of the U.S. Midwest and Northeast and Canada, Pataki said Friday.
After the last major outage blacked out the city in 1977, a system of safeguards was put in place to prevent such events.
"Where did this happen? How did it happen?" Pataki said. "Given the safeguards that were put in place ... why did the system fail?"
Pataki said earlier reports that a major power plant at Niagara had been the source of the outage were "completely inaccurate."
NERC said in a statement that the problem "appears to have been largely caused by the loss of several major power lines in the upper Midwestern United States."
The Associated Press reported Friday that indications pointed somewhere along Lake Erie in Ohio, citing the industry-sponsored group that monitors the transmission system.
"That's where the information is starting to point,'' Ellen Vancko, a spokeswoman for NERC, told The Associated Press. She said it would take time to pinpoint the cause.
Pataki said for the moment, the state's main focus was getting the power back on. Millions of New Yorkers are still without power, he said.
Parts of four New York boroughs and all of Staten Island had power early Friday, a spokesman for the city's Office of Emergency Management said.
Three deaths have been reported in relation to the outage.
New York Mayor Michael Bloomberg said at least one person died as a result of the blackout, and at least one firefighter was injured. Calls came in for some 3,000 incidents of fire, he said, many from people using candles. Emergency services, he said, responded to 80,000 calls to 911 for help, more than double the average.
In Canada, Ottawa Director of Emergency Services Tony Dimanti said a 15-year-old died from injuries suffered in a fire, and another person died at the scene after being hit by a car during an altercation.
Pataki said power officials were bringing the city online slowly to make certain they don't again overload the circuits
Bloomberg urged New Yorkers to stay home and conserve energy.
New York police reported only four burglaries in the entire city overnight, and said they had made arrests in all four.
No fatalities or major injuries were reported, police said, adding that the city's worst problems were with people stuck in elevators and subways.
In addition to New York, outages were reported in Cleveland, Ohio; Detroit, Michigan; Erie, Pennsylvania; Toronto and Ottawa, Canada; Niagara Falls, New York; Niagara Falls, Ontario; and other cities in Connecticut and New Jersey.
The power was cut in just three minutes, as 21 power plants stopped operating, according to Genscape, a company that monitors the output of power plants. The plants, including 10 nuclear plants, shut down between 4:10 p.m. and 4:13 p.m., Genscape said.
Canadian officials insisted the outage originated in America, but U.S. officials and experts were less certain.
The situation looked grim in Detroit -- it might be late Sunday before power is restored to all of Detroit Edison's 2.1 million customers, said Michigan Gov. Jennifer Granholm.
She said workers were trying to restore power to residents in Detroit, Lansing and Ann Arbor.
Granholm said Detroit Edison told her they can't say when all power will be fully restored but that it might take until Sunday.
Early Friday, Detroit Metropolitan Airport reopened, the governor's office said.
About two-thirds of Connecticut's power customers were back online early Friday, a spokesman for Connecticut's Office of Emergency Management said.
As of 1:30 a.m., Dana Conover said, 100,000 customers were without power statewide, but mainly in an area concentrated in the southwestern corner of the state, bordering New York. At the peak of the outage, the number was about 300,000.
"We looking for 99.9 percent restoration by noon Friday," Conover said.
Rob Glenn with the Ohio Emergency Agency said "it does look promising" as electricity is back on in two counties and has been partially switched on in 13 others. Up to a million customers were without power at the peak of the outage, he said.
In a telephone interview, Glenn said 50 National Guard troops have been deployed with 450-gallon tank trucks to distribute water to areas of greater Cleveland that are having problems after electric water pumps shut down.
"We estimate that close to a million of people lost their water over the course of the evening," Cleveland Mayor Jane Campbell said Friday morning.
She said she expected full power to be restored to Cleveland later in the morning.
Glenn said electricity has been restored to much of Tower City in downtown Cleveland.
New Jersey, Toronto have power
By late night, power had been restored to all but 250,000 of the nearly 1 million New Jersey customers who had lost it, a spokeswoman for Public Service Energy and Gas said.
The customers were in five New Jersey counties -- Essex, Hudson, Bergen, Passaic and Union, said Emma Byrne. She predicted power would be fully restored to the company's customers by late Friday morning.
On the Canadian side of the border, Ontario Hydro was working to re-establish power.
By early Friday, the lights were coming on in central Toronto but remained out in other parts of the city.
"Major sections of the downtown core now have power restored," said journalist Adam Vaughan.
New York's subways will not be operating Friday morning, a New York Transit Authority spokesman said, regardless of the status. The transit authority said it will take at least six more hours once power is restored before the subway system is fully operational.
Buses, however, will be running Friday morning.
Amtrak officials said that New Jersey transit trains and some Amtrak service between New York and Washington was operational, but service to Boston and Albany was not running.
The Long Island Railroad will also be offline Friday morning.
All airports shut by the blackout were fully operational Thursday night, although they were still showing the effects of the outage, aviation officials said.
Metropolitan Transit Authority Spokesman Mark Groce said passengers had been evacuated from nearly all of the approximately 500 subway trains that were running when power was lost.