WASHINGTON, D.C. -

Federal energy regulators on Tuesday blamed inadequate planning and lack of grid coordination for a September 2011 blackout that affected millions of people in Southern California -- including the eastern portion of the Coachella Valley -- Arizona and Mexico's Baja California.

The staff of the Federal Energy Regulatory Commission and North American Electric Reliability Corp. issued the findings of their investigation.

FERC Chairman Jon Wellinghoff said the report shows the growing need for more coordination of grid operations in the West.

The power outage affected 7 million people on Sept. 8, 2011.

Utility officials previously said an employee's work on a transmission line at an Arizona substation was the first action in the chain of events, but it should not have triggered such a massive blackout because the system is built to quickly compensate
for such glitches.