lectrical problems were percolating around northern Ohio for at least an hour before failures in transmission lines there joined a chain of events leading to North America's biggest blackout, according to new details provided by utilities to a task force investigating the event.

The information, including second-by-second data on the conditions of some lines and records of conversations among grid operators, was presented to American and Canadian government investigators at a meeting in Newark on Friday, said several people who attended the session and industry officials who were told what was discussed there.

Government investigators and industry groups would not comment on the details of the inquiry.

But all signs point to northern Ohio as the place where troubles began on Aug. 14. In particular, those signs point to the failure of two big transmission lines in suburbs south of Cleveland, at 3:06 and 3:32 p.m.

The second of those two lines went out with a bang — actually, two. The line, which like its counterpart, was owned by FirstEnergy Corporation, overheated and sagged where it bisects the village of Walton Hills, until electricity arced into nearby trees.

Marlene Anielski, the village mayor, said she was taking something out of her car at her home a quarter-mile away. She described "this big boom, a couple seconds," and then another boom. "I was the second phone call in to the police department."

She said appliances in nearby homes were smoking and a crew from Nelson Tree Service, an Ohio company that had the FirstEnergy contract to keep trees from growing toward transmission lines, was running from house to house urging people to get out. Then at 4:10 p.m., the power went out.

"I figured something bigger must be going on," Ms. Anielski said. "Then the phone calls starting coming in that New York's power was out. Detroit was out."

Ms. Anielski said she knew of no problems in the area in recent years related to the lines or nearby trees.

In the search for possible causes of the blackout, questions have been raised about everything from tree-trimming efforts to computer viruses. Early this year, a virus affected a computer system at FirstEnergy's Davis-Besse nuclear power plant, but environmental groups say the computers were not linked to reactor operations.

Many power experts say the most likely cause of the blackout was a delay in finding ways to compensate for the lost line capacity in Ohio, followed by the failures of systems or procedures meant to keep one region's power troubles from becoming everyone's problem.

An update on the status of the government investigation may be provided as early as today by the Energy Department, which is directing the analysis in partnership with the Canadian Ministry of Natural Resources.

But an Energy Department spokeswoman, Corry Schiermeyer, said that far more data had to be collected and put in sequence before any conclusions could be drawn, and that people "should not have any kind of prejudgment in the meantime."

Still, many industry officials and energy experts have already identified Ohio and the transmission territory of FirstEnergy as a prime piece in the blackout puzzle.

On Friday, American Electric Power, another utility company in Ohio, released a detailed timeline of events on its lines, which connect from the south with FirstEnergy's system. Officials at American Electric said their system, detecting growing problems to the north, automatically severed connecting lines to cut off a drain of electricity.

On Friday, American Electric officials said they were providing more information to the international task force, and they briefed regulatory agencies in states served by the company.

Electrical links between Ohio and Michigan remain another focal point in the investigation. On these lines, devices called relays are supposed to cut the power at the first sign of big fluctuations.

But investigators and experts say that some problem — perhaps faulty settings on the relays or a delay in power managers' decisions — allowed Ohio's troubles to streak into Michigan.

That is where the dominoes really started to topple, experts say. As power plants in Michigan shut down in quick succession, Michigan started draining power from Ontario, abruptly reversing what had been an outward flow of electricity toward Canada.

The other vital puzzle piece remains the capacious power lines connecting Ontario and New York. Those links shut down as New York was exporting power to Canada. The abrupt shutdown left New York's lines overloaded, prompting plants in the state to start shutting down.

Those shutdowns then caused fluctuations in the electricity that prompted more New York plants to protect themselves by cutting off from the grid. And by 4:11 p.m., communities from Walton Hills to New York City found themselves girding for a long night in the dark.

Some industry officials participating in the inquiry said the data should provide more answers once the timelines kept by different regions and companies have been synchronized.

Elected officials from Ohio to New York are clamoring for answers. The New York State Assembly plans to hold a hearing in Albany on Wednesday, the first of several on why things went wrong in most of the state.