The huge power cut which left up to 50 million people without electricity in North America probably started in Ohio, industry watchdogs have said.
The power cut left many struggle to continue their daily lives
Investigations are focusing on why three faults picked up in Cleveland were not brought under control.
Meanwhile consumers are being urged to conserve electricity as parts of the United States and Canada recover from the worst power cuts in North American history.
The two governments are setting up a joint taskforce to find out what caused the power failure on Thursday, shutting down parts of their inter-connected grid within three minutes.
Electricity supplies have been restored in most areas, but officials are warning of the possibility of rolling blackouts to prevent the grid becoming overburdened as it stabilises.
The North American Electric Reliability Council (Nerc) said investigations into the cause of the power cut were focusing on why the three faults picked up in Cleveland were not brought under control.
"We are fairly certain that the problem started in
Ohio," said Michael Gent, chief executive officer of Nerc.
And Andre Parker, vice president of Hydro Ottawa, told local media in Montreal: "The latest information we have indicates that there was a break in one of the major US circuits, in Cleveland."
"Those circuits are all interconnected, and that sparked a
high-voltage chain reaction affecting Toronto, Detroit, Ottawa and New York," he added.
FirstEnergy utility company said alarm systems which might have alerted engineers to the failed lines were broken, but that functioning back-up systems had been in place.
The taskforce examining what went wrong will be jointly headed by US Energy Secretary Spencer Abraham, and the Canadian Minister of Natural Resources, Herb Dhaliwal.
US President George W Bush has said the blackout was a "wake-up call" to modernise an antiquated system.
The blackout affected eight US states and parts of eastern Canada, including in major cities like New York, Detroit, Cleveland, Toronto and Ottawa.
Power is now up and running in nearly all areas after nearly 30 hours without service, but problems in some areas were expected to run into Sunday and even next week.
Although the supply has been restored to much of Toronto, some residents in high-rise blocks of flats have complained they are still without power and water - and have been unable to flush their toilets since the crisis began.
Avoid 'frivolous use'
Ontario's Premier Ernie Eves has urged everyone to avoid what he called "frivolous use" of power.
"It is very aggravating to go by a lit-up billboard when others are going without electricity," he told reporters.
"There are people out there, especially the elderly, who need that electricity."
Residents in Detroit and in Cleveland have been urged to boil drinking water amid concerns that sewage may have contaminated the water systems.
No calculation has yet been made of how much the power cut has cost but it is likely to run to billions of dollars.
Shops lost business as customers stayed at home and supermarkets were forced to throw away huge piles of food which spoiled when fridges and freezers shut down.
Work was suspended at plants belonging to the three biggest US car manufacturers on Friday, affecting thousands of employees, while hundreds of flights were cancelled.