Florida's largest utility couldn't estimate on Wednesday how long it might take to determine why a problem with a single switch at a substation led to blackouts across the state a day earlier that cut power to millions of people.

Florida Power & Light spokeswoman Amy Brunjes said a thorough investigation is being conducted but the company couldn't say whether it would be days or weeks before an answer is found.

"As long as it takes to get to the cause," Brunjes said. "We're conducting an extensive investigation."

The massive outages Tuesday began at about 1 p.m. when a switch failed, like a short circuit, at a Miami-area substation, followed by a fire at the facility, FPL said.

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Two nuclear reactors at the company's Turkey Point facility south of Miami automatically shut down after sensing problems in the grid, eventually leading to nearly a fifth of Florida's population being in the dark from Miami to Daytona Beach.

The outage affected up to 3 million people at its peak, but power was quickly restored to most parts of the state by early evening. Authorities reported no injuries.

FPL president Armando Olivera said the company was puzzled as to why such an equipment failure "caused the kind of widespread outage that we saw."

"That's the part that we don't have an answer for yet," Olivera said Tuesday, adding that human error had not yet been ruled out.

In total, the state temporarily lost the ability to generate about 2,500 megawatts of power, or about 5 percent of the generating capacity Florida uses on a peak use day, said Linda Campbell of the Florida Reliability Coordinating Council, which helps oversee the state's electricity supply.

The Turkey Point reactors generate about 1,400 megawatts of power, according to FPL, more than half of the capacity temporarily lost on Tuesday.

Authorities said there were no safety concerns at the nuclear plant, and the outages were not related to terrorism.

Officials said Miami International Airport, the Port of Miami and the region's train and bus systems were working normally, although some places briefly relied on generators, including a few area hospitals.

At one Starbucks Coffee Co., employees began handing out sandwiches they feared would go bad. Nelson Suarez, 35, enjoyed the free lunch. "I can't work anyway since all the power is out, so at least something good came out of this," he said.

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