First the prevailing thought was the power outage affecting 7 million people last month in the Southwest U.S. and part of Mexico was the fault of a utility worker doing a minor repair job. Not so, say investigators.

While they are not totally sure of all the causes, they do know one person did not set off a chain reaction of failures throughout California, Arizona and Mexico.

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Federal investigators and officials from California and Arizona utility companies said they are still investigating the cause of the Sept. 8 blackout.

The utility worker’s repair of a transmission line at an Arizona substation was the first action in the chain of events, it should not have triggered a massive blackout because a complex system involving five electric grids should quickly compensate for such glitches and prevent the problem from spreading, utility officials said.

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Investigators found at least 20 problems took place across the five grids — including one in Mexico — within an 11-minute period, said Stephen Berberich, president and chief executive of the California Independent System Operator, or ISO, which operates the state’s wholesale power system.

“This blackout should not have happened,’’ Berberich said during an oversight hearing of the California State Assembly Committee on Utilities and Commerce, and of the Joint Legislative Committee on Emergency Management, held in San Diego.

Donald Robinson, president and chief operating officer of Arizona Public Service Co., said it was wrong to pin the blame solely on a utility worker at his company’s substation.

“There are still many things we do not know that happened on those days,’’ he said.

But the probe over the past two months has revealed one fact, Robinson said: “This event was not caused by the actions of a single utility worker. That is an unfortunate perception that came out early in the process. The system is built to withstand an event like that.’’

The utility companies involved, under ISO’s direction, formed a joint task force to investigate. Berberich said it could take up to a year before investigators find the exact cause.

The Federal Energy Regulatory Commission and the North American Electric Reliabilty Corp. also have opened a joint inquiry into the outage.

The outage knocked out traffic lights, causing gridlock on the roads in the San Diego area. Two reactors at a nuclear power plant up the California coast went offline after losing electricity. Nearly 3.5 million gallons of sewage spilled into the water off San Diego, closing beaches in the eighth-largest U.S. city.

The National University System Institute for Policy Research has estimated the outage cost the San Diego-area economy more than $100 million.

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