Washington (CNN) -- A massive power outage in September 2011 that left millions of people in California, Arizona and Mexico in the dark was the result of weakness in operational planning and situational awareness, according to an independent report.
The Federal Energy Regulatory Commission, an independent agency that regulates the interstate transmission of electricity, natural gas, and oil, and the North American Electric Reliability Corporation, an industry trade group that enforces national reliability standards, released their findings in a report Tuesday after an eight-month investigation.
The blackout started at a substation in North Gila, Arizona, September 8 and set off a series of failures, eventually leaving 2.7 million customers in southern California and Arizona in the dark.
Officials say it was the loss of one 500 kv transmission line in Arizona that triggered the rolling blackout.
"That line itself did not cause the blackout, but it did initiate a sequence of events that led to the blackout, exposing grid operators' lack of adequate real-time situational awareness of conditions throughout the (region)," the report said.
The FERC-NERC report analyzed how the power outage occurred and detailed a number of recommendations to avoid similar situations in the future.
The inquiry used computer modeling and simulations to determine that "entities responsible for planning, operating and monitoring the bulk power system were not prepared to ensure reliable operation or prevent cascading outages."
The report recommends operators "improve their situational awareness through improved communication, data sharing and the use of real-time tools."
FERC Chairman Jon Wellinghoff said the report "highlights the growing need for more coordination of grid operations in the West."
Utility officials said the short circuit occurred when an Arizona Public Service Co. worker was handling a capacitor at a substation in North Gila. That knocked out a major transmission line that carries power to California.
The blackout brought business in the region to a halt. Trains and trolleys stopped in their tracks, transportation authorities said. San Diego International Airport shut down for a time. Businesses had to close. People were stuck in elevators.
Mexico's border cities, including Tijuana, and parts of Mexico's Baja California and Sonora states were also affected by the outage, Mexican authorities said.