The largest power outage in U.S. history blacked out the Northeast yesterday afternoon, affecting an estimated 50 million people, including residents of New York City, New Jersey, Connecticut, Ottawa, Toronto, Cleveland and Detroit.

A New York state official said the outage came when the Niagara-Mohawk power grid failed just after 4 p.m. Eastern time, CNN reported. Officials believe the cause is not related to terrorism.

“This is most likely a natural occurrence,” said New York City Mayor Michael Bloomberg.

Bloomberg said at a news conference just before 6 p.m. it would take hours rather than minutes to restore the system, noting power was starting to come back in some areas of the Northeast.

“There is no evidence of any terrorism whatsoever,” the mayor said.

Canadian Prime Minister Jean Chretien’s office said a lightning strike on a power plant near Niagara Falls, N.Y., was the cause, according to reports by Reuters and WKBW in Niagara Falls.

The prime minister’s spokesman, Jim Munsen, cited security sources on both sides of the U.S.-Canadian border who believe the lightning struck the New York side. U.S. officials, however, have not confirmed this.

Later, another Canadian official blamed a fire at a Pennsylvania power plant, the Associated Press reported.

“The fire started at a nuclear power plant in Pennsylvania,” Canadian Defense Minister John McCallum told reporters. “The origin of the problem is a fire in Pennsylvania which has caused a cascading effect in the rest of the region.”

Maria Smith, a spokeswoman for the Pennsylvania Emergency Management Agency refuted the Canadian’s assertion.

“That is absolutely not true,” she told AP. “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.”

The entire city of Toronto was affected by the outage, according to MSNBC television. The Canadian Press reported Ottawa’s Parliament Hill was without power, except for scant emergency lighting.

Four hours later, power was coming back online stage by stage, including in Detroit, parts of the Bronx and New Jersey, but federal officials said it might not be fully restored until this morning.

President Bush, attending a fund-raiser in California, told reporters the incident underscores the need to modernize the nation’s electrical grid.

The president said the federal government has offered help.

“We’re prepared to do anything we can upon request,” he said.

But he noted local authorities have responded well, and the emergency system has worked.

Long trip home

The blackout, which briefly knocked television networks off the air, sent thousands of New Yorkers streaming into the streets.

CNN reported a fire at the Consolidated Edison plant on 14th Street in Manhattan. However, Bloomberg explained the thick black smoke is the normal result of a programmed shutdown of the plant’s boilers.

The New York Port Authority said, according to ABC News, there was no train or subway service in the city after the blackout. Thousands of people were believed to be stuck in stopped subway cars with no air conditioning.

With the blackout striking near the end of the work day, thousands also were stuck in elevators in city high-rise buildings. Outside, traffic lights were out, stalling traffic, and pedestrians filled the roadways.

The reported high temperature yesterday in New York City was 91 degrees.

FAA officials told NBC News air traffic control was operating through the use of backup power systems.

WNBC reported all major airports were blacked out. ABC News said planes were allowed to land but not take off.

New York City Hall was operating on a backup generator.

Bloomberg said at his press conference President Bush’s chief of staff Andrew Card and New York Gov. George Pataki both have offered aid.

The city was adding extra buses and opening some bridges to pedestrian traffic to help workers get home, said Jarrod Bernstein of the New York City office of emergency management in an interview with ABC News.

“Residents of the New York area are pulling together as they always have during an emergency,” Bernstein told ABC’s Ted Koppel. “And we will get through this.”

Detroit Mayor Kwame Kilpatrick told Koppel last night all power was out in the Michigan city, but traffic was moving smoothly and buses were functioning. City suburbs were gridlocked, however, he said, because most workers went home at the same time.

Ohio Lt. Gov. Jennette Bradley said in an ABC interview power had been restored to some areas of Toledo and other parts of the state as of about 6:30 p.m. Eastern, but not Cleveland.

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