The blackouts began around 4pm local time, although officials could not pin down exactly what caused them. Some reports traced the cause to a localized outage in a New York State-area power plant, which destabilized the power grid as far west as Detroit and Cleveland and as far north as Toronto and Ottawa. The Niagara Mohawk Power Grid, which supplies electricity to much of the northeast United States, was already overloaded on the hazy, humid afternoon when the outage occurred.
In scenes likely repeated throughout the cities affected by the power failure, thousands of New Yorkers streamed out of buildings and into the streets as blackout-affected streetlights snarled traffic. Thousands more were believed stuck in the powerless subway system or in elevators as rescue workers tried to reach them. "Right now the power outage is affecting all of our operations," a spokesman for the New York Port Authority told Reuters news service. "We have no buses, no trains, no subways running." However, there was no looting reported as of late afternoon, and few signs of panic according to local news sources; most people seemed to be calm, if bewildered by the sudden loss of power. New York Governor George Pataki said that as many as half of New York State's 19 million residents could be without lights, air conditioning or electricity.
The Federal Aviation Administration grounded planes at all three New York area airports and in Cleveland because the blackout affected security screening facilities, although most air traffic control operations were kept online by backup generators or alternate power sources as flights were rerouted away from the affected area.
Several U.S. nuclear power plants went offline, according to the North American Electric Reliability Council, an industry group responsible for the security of the nation's electric grid. The plants were not physically affected by the blackout: with the grid down, the power they were producing had nowhere to go—electricity is difficult to store—so operators took the plants offline.
By 6 p.m. the power was being restored in parts of the affected area. A spokesman for the Federal Energy Regulatory Commission said the agency had been told by the North American Electric Reliability Council that the power outage has been "contained."
The New York Stock Exchange announced plans to open on schedule Friday, using emergency power, if necessary.
New York Mayor Michael Bloomberg said at a news conference that power was being restored to the northern and western areas of the region affected by the blackout, although he also said that it would be "hours not minutes" before full power would return to the area.