U.S. officials have blamed Canada for a massive power failure hitting major cities in both countries Thursday, but Canada says the trouble started in the United States.
As power began to return to many affected areas early Friday, there was no official cause established for the blackout.
On Thursday, the office of Canadian Prime Minister Jean Chretien said a suspected lightning strike hit a power plant in New York state.
Officials at the Niagara Power Plant told the CNN television network there was no lightning reported Thursday.
Meantime, the mayor of New York city said Friday, the event triggering the outage occurred in Canada. Michael Bloomberg told a press conference that Canadian energy workers failed to act to stop the problem from spreading.
U.S. energy analysts have also said the problem may have started at a power station in the Midwestern state of Ohio.
President Bush said Thursday there were no signs that terrorism was to blame, but promised to investigate the causes.