The heart of North America’s automotive industry was shut down from about 4pm eastern daylight time on Thursday afternoon by cascading electric power cuts that struck the northeastern United States and southern Canada, Reuters reported.

Citing officials, Reuters said it was the largest power cut ever to hit the United States – there were also massive blackouts in 1977 and 1965.

Reuters said that President George W. Bush, ruling out terrorism as the cause of the blackout, promised to investigate the cause and prevent a recurrence.

Officials in south-eastern Michigan told the news agency late on Thursday evening that power was returning to some areas, but it could take much of the weekend before all power was restored. Lights also began to flicker on again in Ontario just before midnight, eastern daylight time, some seven hours after the cut hit, a separate report said.

CNN described the situation in Detroit as looking grim and, citing Michigan governor Jennifer Granholm, added it might be late on Sunday before power is restored to all of Detroit Edison‘s 2.1 million customers.

Granholm reportedly said workers were trying to restore power to residents in Detroit, Lansing and Ann Arbor and added that Detroit Edison told her they can’t say when all power will be fully restored but that it might take until Sunday.

“We have plants in all the impacted areas and we’re still assessing the impact,” GM spokesman Pat Morrissey told Reuters.

Ford spokesman Ed Lewis told the news agency 23 of Ford’s 44 plants in North America went down, including those assembling vehicles, stamping body panels, and making engines and other parts.

Chrysler spokeswoman Debra Nelson told Reuters nine of its 13 North American final vehicle assembly plants were hit by the outage, adding that some of the plants may not have lost power, but could have shut down as a precaution.

Separately, CBS Marketwatch said the cuts stopped power supply to Honda’s sole facility in Canada though the Japanese car maker has two other manufacturing facilities in the US which weren’t affected by the blackouts.

The plant, located in Alliston, Ontario, produces the Honda Pilot, Odyssey and Civic sedan as well as the Acura MDX and Acura 1.7EL., CBS said. The blackouts, which occurred late Thursday afternoon, forced the plant to suspend production on a late shift starting from 4:30 p.m.

“I don’t now when production will resume. It is a very confusing situation right now,” Honda spokesman Shigeki Endo told CBS Marketwatch.

The report added that Alliston is scheduled to produce 390,000 vehicles this year and a one-day suspension would cut production by around 1,600 units. But since the plant closes on the weekends, there will be little impact if power supply returns to normal at the start of next week, the report said.

CBS Marketwatch said officials at Toyota, Nissan and Mazda were not immediately available to comment as the companies are closed this week for the “Obon” holidays.

Reuters said the massive power cut hit Michigan, New York, Ohio, New Jersey and parts of Canada, where much of the North American motor industry’s plants and offices are concentrated. The headquarters of all three car makers lost power; GM employees had to walk down from their skyscraper by the Detroit River.

“We are sitting here in the dark,” Chrysler spokesman Dave Barnas told Reuters from the company’s headquarters in Auburn Hills, Michigan.

The news agency said road traffic was also snarled at the Ambassador Bridge, the major freight connection between Detroit and Ontario, Canada.

Reuters noted that all three car makers have more than two months’ worth of new vehicles in stock and at dealers, making shortages unlikely. The news agency also noted that an unplanned plant shutdown can hit earnings directly, since car makers book revenues when a vehicle is built, but short interruptions in production can usually be made up by running overtime.