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SDG&E President Mike Niggli said Thursday night that an Arizona Public Service employee triggered the cascading power outage that affected 1.4 million households and businesses.

The blackout—which was triggered when the employee carried out a procedure in the North Gila substation northeast of Yuma—is now the focus of an investigation of "the sequence of events that led to the outage and establishing practices and procedures to ensure that outages such as the Sept. 8 event are not repeated," according to an SDG&E press release.

SDG&E officials said the transmission outage in Arizona ultimately caused the San Onofre Generating Station to go off and "as a result, SDG&E did not have adequate resources on its system to keep power on across its service territory." The blackout was the largest in the company's history.