NEW YORK (CNN) -- The cause of a massive power cut that paralyzed New York, much of the northeast U.S. and parts of Canada -- putting 10 nuclear plants out of service -- was shrouded in confusion with America and Canada pointing the finger at each other.

In Toronto, Canadian Defense Minister John McCallum said it was a power outage at a nuclear plant in the United States. U.S. officials and experts were less certain.

The New York Independent System Operator (NYISO) said a fault occurred west of the Ontario power system at 4:11 p.m. Thursday, and "it is not believed" that the massive outage that ensued originated in New York State.

"Reports that the occurrence began in Niagara are not supported by the facts currently available," the NYISO statement said.

The North American Electrical Reliability Council (NERC) -- an industry group dedicated to maintaining the integrity, security and reliability of the nation's power grid -- said the cause of the outage was unknown, but that a total of 10 nuclear plants went off-line as a result.

NERC said Perry Nuclear plant not far from Cleveland, one of just two nuclear plants in the state, went off-line at a time that "major transmission lines were out of service."

In all, the organization said, 10 nuclear plants "went off-line, probably as a result of fluctuations in frequency." It discounted the possibility of terrorism but did not say where the other plants are.

"They came off-line because of the disruptions," said NERC spokeswoman Ellen Vancko. "Nuclear plants shut down to protect themselves. That just exacerbated the situation."

Dale Wedge, director of emergency services for Geauga County east of Cleveland, said: "The outage is what caused the power plant to scram. They get the power from off-site to be able to run.

"When they lost power from the grid, that's when the power shut down."

In New York, focus of major rush-hour-and-beyond disruption caused by the blackout, Gov. George Pataki said he did not know what caused the blackout in the first place.

The Niagara Mohawk power grid, which distributes power to the northeast corridor and into Canada, also said the cause of the outage was unknown.

"They haven't found the cause yet," said Amy Atwood, a spokeswoman for National Grid, which owns Niagara Mohawk. "Everything we're aware of right now is just speculation."

In a Toronto teleconference with reporters late Thursday, McCallum said in response to questions about what caused the blackout: "My understanding is it was a nuclear power plant."

He said he was getting his information from U.S. military officials and that they assured him the outage "was not terrorism or sabotage."

The information came shortly after Canadian officials first said the cause was lightning at a power plant in New York, and then quickly backed off that theory, saying it was a fire at a Con Edison plant in New York.

McCallum acknowledged he was aware of those same reports, but he said he had learned the latest information from U.S. counterparts about 10 minutes before the teleconference.

"We're going by the latest information we have received from our U.S. counterparts," he said. "We are told this as a fact."

He said he did not know which power plant suffered the outage or how it was caused, but he believed it was in Pennsylvania.

Maria Smith, a spokeswoman from the Pennsylvania Emergency Management Agency, said it did not happen in her state.

"All our power plants have been operating at 100 percent power," Smith said. "We've had no problem with our nuclear plants in this state, or no issues whatever."

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