The effort to nail down the source of the massive blackout turned Saturday to northern Ohio, where the power system is operated by FirstEnergy Corp. (search), the nation's fourth-largest investor-owned utility.

Akron-based FirstEnergy has 16 power plants and an annual revenue of more than $12 billion, with a service area that stretches from Ohio to New Jersey.

The company is accustomed to scrutiny.

Its Davis-Besse (search) nuclear plant east of Toledo has been shut down since February 2002, when it was closed for maintenance. A month later, a leak was found that had allowed boric acid (search) to eat nearly through the 6-inch-thick steel cap on the plant's reactor vessel.

On Saturday, a top investigator said the failure of three transmission lines in northern Ohio likely started Thursday's blackout that swept into eight states and Ontario.

Investigators were "fairly certain" that the problem started in Ohio, said Michehl Gent, head of the North American Electric Reliability Council (search). "We are now trying to determine why the situation was not brought under control."

The council is a nonprofit, industry-sponsored group that is supposed to oversee power line reliability.

Ohio regulators also said the blackout may have started in their state.

FirstEnergy, which officials said owns four of the first five lines that failed, said a system that is supposed to flash a red warning on computer monitors at the company's control center was not operational when the lines began failing Thursday afternoon.

It was not immediately clear whether that impeded efforts to isolate the local line disruptions. Company spokesman Ralph DiNicola said a functioning backup alarm at the Midwest Independent System Operator, a nonprofit power pool that oversees the region's electrical grid, was in place.

At the Midwest ISO, spokeswoman Mary Lynn Webster said she did not know when workers noticed the FirstEnergy lines were disabled and what, if anything, they did about it.

FirstEnergy executives met privately Saturday to determine the extent of the utility's role in the outage. Lunches were brought in and entry into FirstEnergy's downtown brick headquarters was strictly controlled.

Earlier this month, a U.S. District Court judge in Ohio ruled that FirstEnergy should have determined that changes at one of its coal-fired plants would increase overall pollution emissions. A second trial will determine penalties.

U.S. Rep. Dennis Kucinich, a Democratic presidential candidate from Cleveland and a FirstEnergy critic, said he was closely monitoring the situation.

"If press reports are accurate, and FirstEnergy once again failed to comply by federal standards, then I believe serious questions must be asked about FirstEnergy's ability to effectively serve," he said.

Cleaning up the Davis-Besse plant has drained FirstEnergy's finances. It lost $57.9 million in the most recent quarter, primarily because of Davis-Besse costs.