The electricity supply is returning to areas in a large portion of the Northeastern U.S. and Canada Friday morning after a power failure blacked out millions of homes and offices Thursday.The North American Electricity Reliability Council (NERC) Friday said in a statement that approximately 41,100 megawatts (MW) of load was restored to the grid system in the area. That represents just over a two-thirds of the 61,800MW of load that the grid had been handling at the time of the power failure, it said.NERC also said that it could take several days for the power supply in the region to fully return to normal. While fossil-fired power plants could be restarted in a matter of hours, nuclear power plants could take longer to come back online. A total of nine nuclear power plants in New York, New Jersey, Ohio and Michigan were shutdown as a result of the problems, said the U.S. Nuclear Regulatory Commission.The failure began at around 4:10 p.m. Eastern Daylight Time after an as-yet unidentified problem caused an imbalance between the amount of power being generated and consumed, and safety mechanisms built into the power system began shutting it down, said several power company and local government officials during a series of televised press conferences.Cities including New York, Detroit, Cleveland, Ottawa and Toronto lost the majority of their power supply.Some early reports pointed blame at a lightning strike at a power facility on the U.S. side of the border in the Niagara Falls area although the local power company said it had no immediate evidence of such an occurrence and called the reports "speculative.""One thing I think I can say for certain is that this was not a terrorist act," U.S. President George Bush told reporters in San Francisco, according to a transcript of comments made available by his office.Other officials too discounted the possibility of terrorism and appeared to contradict themselves by saying they had no idea what caused the problems. Challenged on this during a news conference, William Edwards, the president and CEO of Niagara Mohawk Power said he based the view on the way the power system shut down."The system behaved in a very normal fashion, as designed, by shutting loads and putting generators offline," he said.Early Friday morning NERC said the cause of the problem may lie further west. In a statement it said the problems "appear to have been caused by the loss of several major transmission lines in the upper Midwestern U.S." and added that investigations are continuing.While the impact on most homes and businesses in the affected area was immediate and complete, the impact on telecommunications systems and the Internet was less severe.Cellular carriers reported disruption to services although battery backups and generators enabled many to quickly restore services. Problems continued due to a surge in demand that made placing and receiving calls difficult, said operators both north and south of the border.Internet monitoring company Keynote Systems said it saw no major problems on the Internet backbone or at major Web sites in the Northeast. Several news sites had minor performance problems initially, but this was attributed to the heavier than normal demand that often accompanies a major news event.