COLUMBUS, Ohio - Utility companies were expected Tuesday to tell state regulators why they marked certain documents confidential and to justify prohibiting their release.

The documents are among several required to be filed with the Public Utilities Commission of Ohio. The PUCO has received several requests from news organizations for documents relating to the blackout earlier this month.

The documents were filed before the Aug. 14 blackout that darkened homes and businesses in eight states and parts of Canada. The blackout affected 50 million people, shut down more than 100 power plants and knocked Cleveland's water supply off line.

U.S. and Canadian investigators are focusing on failures of a power plant and lines owned by FirstEnergy Corp. in Ohio, including one transmission line that sagged into a tree.

Acting on behalf of the PUCO, the Ohio attorney general's office informed five utilities on Friday that they needed to defend their contention that the documents should not be made public.

The utilities are FirstEnergy of Akron; American Electric Power Co. of Columbus; Dayton Power and Light; Cinergy Corp. of Cincinnati; and Pennsylvania-based Allegheny Power.

"At this point, it is not clear why the company marked some of this information as confidential when submitting it," Steven Nourse, the attorney general's top public utilities lawyer, said in the form letter sent to the companies.

The burden is on the companies, PUCO legal director Paul Duffy said Monday.

"We're not seeking their permission, we're asking them for their legal argument as to why it constitutes a trade secret," Duffy said. "If they don't meet burden, we're not going to protect it."

PUCO staff do not believe the documents contain trade secrets, Duffy said.

FirstEnergy spokeswoman Ellen Raines said the company has a tradition of submitting certain documents to the PUCO marked confidential in whole or part.

"It could be for security reasons, it could be for financial disclosure reasons," Raines said Monday.

For example, portions of the company's long-term forecast were marked confidential for security reasons.

FirstEnergy officials were reviewing the documents and expect to report to the PUCO on Tuesday on whether any can be released without raising security issues or potential violations of Securities and Exchange Commission guidelines, Raines said.

AEP spokesman Pat Hemlepp said the company is reviewing the documents for anything it considers truly confidential.

"If there is anything like that, we might ask for continued confidentiality," Hemlepp said.

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PUCO: http://www.puc.state.oh.us/