ORLANDO, Fla. -- Power executives are still in the dark about how a glitch at a substation triggered a blackout that cut power to millions across south Florida.

The Nuclear Regulatory Commission said that the two Florida Power & Light nuclear reactors at its Turkey Point power point about 25 miles south of downtown Miami automatically shut down around 1 p.m. Two other power plants farther north in the state, the Crystal River reactor and St. Lucie twin reactors, continued to operate, although officials at those two facilities noticed the grid disturbance.

"We don't know whether the grid disturbance caused the units to shut down or that their shut down caused the grid disturbance," said Kenneth Clark, a spokesman at the NRC regional office in Atlanta. He said the two reactors were automatically shut down.

The outages have no connection to terrorism, Homeland Security Department spokeswoman Laura Keehner said. No foul play was suspected, Miami-Dade County Mayor Carlos Alvarez said.

"There are no safety concerns. The reactors shut down as designed," said Clark in a telephone interview. He said both reactors continued to have offsite electric power. He said two coal-burning power plants at Turkey Point also shut down.

Florida emergency management officials said the outages cut power to about 3 million people, although FPL said the number was closer to 800,000.

FPL estimated power should be restored by 6 p.m. A spokeswoman who spoke briefly to a reporter in the lobby of the company's Juno Beach headquarters did not have any additional information.

The outages began shortly after 1 p.m., though power in some affected areas had been restored an hour later.

In Central Florida, several county sheriff's offices confirmed power outages at schools and traffic signals across their jurisdictions.

Orange County reported power outages in southwest areas and at several schools, including Aloma, Dommerich, Lake Sybelia, Lake Weston, Riverside and Zellwood elementary schools; Maitland Middle School; Boone, Jones and Wekiva high schools and Silver Star Center.

Police agencies also manned several major intersections throughout Brevard County after power was cut to lights.

At least seven Brevard schools were affected by the outage, and Florida Institute of Technology, located in Melbourne, canceled classes for the rest of Tuesday because of the outage.

The outages affected Melbourne, Merritt Island and Titusville, officials said.

Officials in Volusia County said dozens of traffic lights were out.

"I don't have a handle on whether we're experiencing residential or commercial outages," said Gary Davidson, Volusia sheriff's spokesman. "I know we're receiving reports of traffic lights out virtually throughout the county, from DeLand, Deltona, Ormond Beach, South Daytona to Debary."

The outage struck at least two polling sites but did not halt voting in special elections, officials said.

The outage also affected service in Kissimmee and left nearly 7,600 customers without electricity, according to Kissimmee Utility Authority officials.

Two Osceola County schools, Boggy Creek Elementary and Parkway Middle, were affected by the outage, but power was restored after about 30 minutes, officials said.

Outages also hit the southeast portion of the state, including Miami, but were also reported in the southwestern and northeastern parts of the state as well as in the Florida Keys.

Several Miami-area hospitals switched to backup generators when the power went out. Miami-Dade schools were scheduled to be dismissed on time, and officials said school buses would be running.

In Miami's western suburb of Doral, disappointed office workers sat under an awning at a strip mall, unable to buy lunch nor go anywhere else due to a sudden downpour.

Nearby, Panera Bread bakery servers enjoyed the unexpected smoking break at the height of the midday rush-hour, while their manager grumbled over lost sales.

Down the block at Starbucks Coffee Co., employees began handing out sandwiches they feared would go bad.

Nelson Suarez, 35, a manager for Asia sales at World Fuel Services, 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.

Jaime Hernandez, a spokesman for Miami-Dade County Department of Emergency Management, said the county is partially activating its emergency operations center. He said no injuries have been reported so far.

By 2 p.m., most of northern downtown Miami appeared to be back to normal operation, including a campus of Miami Dade College and numerous stores and businesses. Traffic lights were out for a short time but appeared to be back in regular operation. In the Florida Keys, spokesman Andy Newman said areas were without power for about 15 minutes, but it was back up as well.

An official at the Miami International Airport says the facility is working on a generator backup but that no airline delays were reported.

In Collier County in the southwestern portion of the state, sheriff's spokeswoman Karie Partington said officials were working to determine the extent of the outages.

The first of Turkey Point's two nuclear power units started operation in 1972. In March 2006, a tiny hole was found in a coolant pipe at the plant. The FBI determined it was vandalism, not sabotage. An out-of-state contractor hired to do routine maintenance was suspected of drilling the hole, the FBI said at the time. The public's health and safety were not at risk, the Nuclear Regulatory Commission said then.

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