Power has been restored to 502,100 customers, as of 5 a.m. Thursday, Florida Power & Light Company said.

But, the power utility company warned that some customers may experience prolonged outages "because portions of the electric system in Southwest Florida will need to be rebuilt rather than repaired," FPL said in a statement.

The company said is still working to restore power to 1.2 million customers.

FPL said its workforce has increased to more than 20,000 people, including mutual assistance from 30 states, as it works around the clock to restore power.

"Hurricane Ian has forever altered the lives of so many of our fellow Floridians and we recognize the road to recovery will be long and challenging," Eric Silagy, chairman and CEO of FPL, said in a statement.

Lee County Sheriff Carmine Marceno told ABC News' "Good Morning America" that Hurricane Ian was very unpredictable, with officials tracking the storm every minute and they didn't know where it would hit.

"This is a life-changing event for all of us. We tracked that storm up the coast of Florida, it was very unpredictable," Marceno said.

He added, "We didn't know where it would hit. I can tell you it came into Lee County strong and it was slow moving."

Marceno said there were fatalities, but he didn't yet know the exact number.

Marceno said they are already assessing the area, but the whole area is "crushed" and people are trapped. Marceno said they have thousands of 911 calls that they are currently answering.

"We still cannot access many of the people that are in need," Marceno said. "It's a real, real rough road ahead."

In Volusia County, on Florida's east coast, authorities said one person has been confirmed dead. A 72-year-old man who went outside to drain his pool was swept away and found dead in a canal behind the house.

President Joe Biden has approved a major disaster declaration for Florida residents in Charlotte, Collier, DeSoto, Hardee, Hillsborough, Lee, Manatee, Pinellas and Sarasota counties.

"Assistance can include grants for temporary housing and home repairs, low-cost loans to cover uninsured property losses, and other programs to help individuals and business owners recover from the effects of the disaster," the White House said.

-ABC News' Justin Gomez

Flooding has hit several parts of the Orlando area, including Seminole, Osceola, Lake counties and other neighboring areas.

ABC News' Orlando affiliate WFTV said that first responders in Kissimmee rescued at least one person from a couple of vehicles that had been partially submerged during a storm surge overnight.

It is unclear how severe the damage is in the area but witnesses reported heavy and sustained wind and rain throughout the overnight hours.

-ABC News' Ahmad Hemingway

Ian remains a tropical storm with winds of 50 mph, but the storm system is expected to rapidly intensify on Sunday as it moves over warm waters in the Caribbean Sea, according to the National Hurricane Center.

The seawater currently carries very favorable conditions for strengthening over the coming days, with temperatures between 85 and 90 degrees Fahrenheit -- the perfect temperatures for fueling a powerful storm as it stays over open water.

Ian is expected to strengthen to a hurricane by Sunday night and into a major hurricane later Monday night as it passes by Cuba.

The NHC currently forecasts Ian to strengthen into a major Category 4 hurricane on Tuesday in the Gulf of Mexico as it heads toward Florida.

Ian is expected to make landfall on the west coast or the panhandle of Florida on Wednesday or Thursday.

Some weakening could occur before the storm system makes landfall, possibly dipping below major hurricane status before impacting coastal communities.

A hurricane warning is currently in effect for the western portion of Cuba, where flooding rains and major storm surge are forecast.

-ABC News' Riley Winch