SAN DIEGO (AP) — Federal officials are expected to release a report on what caused a power outage that affected 7 million people last year in the Southwest U.S. and part of Mexico.

The findings by the Federal Energy Regulatory Commission and North American Electric Reliability Corporation will be announced Tuesday in a joint news conference in Washington.

Utility officials have 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.

The Sept. 8, 2011, blackout affected customers in parts of Southern California, Arizona and Mexico's northern Baja California.