SAN DIEGO — Federal regulators are investigating the massive Southwest power outage that left millions of people in the dark.
The Federal Energy Regulatory Commission (FERC) said Friday it would work with the North American Electric Reliability Corporation, to try to figure out what happened, NBC station KNSD of San Diego reported.
If any regulatory violations are found, the commission could issue fines of up to $1 million per day for every violation, said FERC spokeswoman Mary O'Driscoll.
This inquiry is an effective way for us to protect consumers and ensure the reliability of the bulk power system, said FERC chairman Jon Wellinghoff in a written statement.
The massive blackout lasted 15 hours and up to 5 million customers in Orange County and San Diego, Calif., western parts of Arizona and northern Mexico into darkness, paralyzing freeways, halting flights and closing schools.
San Diego Gas & Electric announced Friday morning that all of its 1.4 million customers had power again after crews worked through the night to make emergency repairs following the accidental outage.
The San Diego Police Department told KNSD that there were no major incidents overnight despite the blackout.Video: Power restored after huge blackout (on this page)
"There was nothing that was blackout-related that was anything more than normal," said SDPD Lt. Andra Brown. "We did not see an increase in violence or activity due to blackout."
However, the blackout also caused a sewage spill that closed some San Diego-area beaches. All public schools in the city were closed Friday as well as local state universities and community colleges.
Officials warned that the electrical grid was still fragile after the outage and asked residents and businesses to go easy on — or even put off using — major appliances, such as air conditioners.
"Conservation will really help reduce the strain," said Stephanie McCorkle at the California Independent System Operator, which manages the power grid.
A decade after California faced rolling blackouts that shutdown everything from ATMs to traffic signals, Thursday's outage raised new questions about the condition of the nation's electricity grid.
Authorities were focused Friday on trying to figure out how a mistake by a single Arizona Public Service Co. worker making a routine repair in Yuma, Ariz., could cascade across the Southwest.
"That work should not have caused this," said Damon Gross, spokesman for the Phoenix-based utility.
"Why it became so widespread is what we are going to work with the other utilities to investigate because the system should have isolated itself," he said. "It's designed to protect itself."
California public health officials in Sacramento even activated the state's Joint Emergency Operations Center to assess the impact.
The move alerts government agencies and other entities to report any medical or public health effects, starts efforts to assess the impact on hospitals and nursing homes and on the public drinking water and regulated food industries.
Night of struggling
The outage came more than eight years after a more severe black out in 2003 darkened a large swath of the Northeast and Midwest, affecting more than 50 million people.
Many spent the night struggling to fall asleep in the high temperatures.
Dan Williams lives in the hot desert of eastern California and usually looks forward to his business trips to San Diego. Not this time, he said, describing his stay at a motel like a camping trip.
"It was hot, there was no air. It was just miserable," said Williams, who slept with the door open.
Several construction workers at a clinic in San Diego stumbled back to work shortly before dawn.
Ed Harris grabbed a beer with his son and watched the traffic congestion from the patio of his San Diego home until he couldn't fend off sleep any longer and had to go back into his roasting residence.
"When I got up, my body left a big bed mark in a sweat ring," he said.
The lights came back on at his home at 2:18 a.m. His wife woke him up to set his alarm clock.
'This was not a deliberate act'
Two reactors at a nuclear power plant along the coast went offline after losing electricity, but officials said there was no danger to the public or workers.
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The outage occurred after an electrical worker removed a piece of monitoring equipment at a power substation in southwest Arizona, APS officials said.
"This was not a deliberate act. The employee was just switching out a piece of equipment that was problematic," said Daniel Froetscher, an APS vice president.
It's possible that extreme heat also may have caused some problems with the transmission lines, said Mike Niggli, chief operating officer of San Diego Gas & Electric Co.
During the night, much of San Diego was in darkness, and all outgoing flights grounded at Lindbergh Field. The airport was open and had power Friday but authorities said some airlines may have cancelled flights.
There were reports of minor traffic accidents as the outage knocked out stoplights during rush hour.
Leah Walden said she saw about five fender-benders on her drive from her accounting job in suburban Spring Valley to a wedding-cake tasting in San Diego.
"People are irritated. They don't want to wait," said Walden.Story: Texas sets record for hottest summer in US
The blackout extended south of the border to Tijuana, Mexicali and other cities in Mexico's Baja California state, which are connected to the U.S. power grid, Niggli said.
In Tijuana, people formed long lines outside convenience stores Thursday, trying to buy ice or take advantage of half-price beer. Many drank it on the streets or in parked cars with speakers booming loud music.
Cars also formed snaking lines at the few gas stations with generators that remained open and traffic snarled streets after traffic lights stopped working.
Jose Padilla Flores was one of the few people who still had electricity Thursday.
He offered to let people watch the telenovela on his television if they bought fried tacos and flavored water from his small restaurant "El Dorado" in the Independencia neighborhood.
"My female neighbors were the first ones to ask if I could let them watch the telenovela," said Padilla Flores, 35. "I thought that was a great idea to promote my business."
San Diego residents poured into the few bars that remained open downtown after dark, some donning reading lights on their heads like miners.
Two men carried flaming tiki torches — usually planted in backyards — to see their way.
NBC News, msnbc.com staff, The Associated Press and Reuters contributed to this report.