Hurricane Irma is moving ashore in southwest Florida after making landfall for the second time on Marco Island close to Naples.
Irma weakened to a Category 2 storm at about 5pm Sunday (local time) as it hugged the coastline moving north towards Tampa, the Daily Mail reports.
The National Hurricane Center said Irma's winds were at 177km/h, just below major hurricane status, as the centre of the still dangerous and wide storm moved further inland late on Sunday afternoon (local time).
It was smacking Naples after coming ashore at Marco Island at 3.35pm (local time).
Areas of Naples are now suffering substantial flooding and swaths of the west coast - as well as lakes, bays and sounds nearby - are under 15ft (4.5m) storm surge warnings. The National Hurricane Center said water levels in Naples rose 7ft (2.1m) in just 90 minutes.
"Pray, pray for everybody in Florida," Governor Rick Scott said on Fox News.
As Irma continued its unprecedented assault on the state, two of the four victims claimed in Florida were identified.
Hardee County Sheriff's deputy Julie Bridges, a mother of one, and Hardee Correctional Institute sergeant Joseph Ossman crashed and died on Sunday around 60 miles (95km) from Saratosa.
While the confirmed death toll is currently just four, the suffering from Irma's onslaught is much wider, with more than three million losing power and swaths of Miami - where two cranes were toppled - left flooded.
The Florida Keys, which is where Irma first made landfall, are now the subject of a huge airborne relief mission.
President Donald Trump declared a major disaster in the state of Florida on Sunday, making federal aid available to people affected by Hurricane Irma in nine counties already hit by the storm.
As the hurricane moves up the west coast, experts have warned that there will be a negative surge of three or more feet (1m) , as water is pulled out into the sea, or into the centres of lakes, bays or sounds.
That might make it look like the hurricane is subsiding - but it's just a prelude to the real surge, which will push huge amounts of water back towards the land.
A storm surge hit Naples at 4.35pm as the eye of the storm got closer to the city, with feet of water suddenly flowing on to streets. But that was a small pre-surge and not the full level.
That kind of flooding is a risk on both sides of the state, but is particularly dangerous on the west coast.
Previous hurricanes have seen dozens killed after people ran out on to beaches to pick up newly stranded fish, only to be caught as the deadly waves flow back on to the helpless crowds.
There are longer-term threats, too, from sewage and other toxins being caught in the surge back on to land.
"We're going to be inundated with unprecedented amounts of water," Tampa Mayor Bob Buckhorn said.
"It's going to stress our storm water and sewer capacity. There's going to be overflows. There's no two ways around it."
That in turn could lead to runoff being sent into Tampa Bay, which means dangers to public health as well as to structures.
Leakage from the radiation and toxins created by the state's phosphorus mining industry - the largest in the nation - as well as from the 51 Superfund sites.
Those are largely old chemical or oil storage facilities that have poisoned groundwater over decades, and are among the most toxic places in the US, according to the Environmental Protection Agency.
Florida's Department of Environmental Protection said it would be testing groundwater as soon as it's safe to do so after the storm. EPA said it will also be on the ground after the storm.
As the nation's eyes turned to follow Irma up the west coast of Florida, the Keys began to take in the immensity of the damage done.
Florida responded with the launch of a massive airborne relief mission by Monroe County Emergency Management, whose director, Martin Senterfitt, called the damage done to the Keys a "humanitarian crisis".
He promised disaster mortuary teams, as well as C-130 cargo planes, which US Air Force special operations pilots are testing flights around the massive storm, the Miami Herald reported.
Also on the mission will be Air National Guard flights of more C-130s, backed up by squadrons of helicopters. They are expected to start arriving early on Monday morning.
The first load will head to Florida Keys Marathon Airport. As it can handle about two C-130 planes at a time, the plan is to land two every two hours, keeping a steady flow of goods.
"The help is on its way," Senterfitt said, adding: "We're going to get more aid than we've ever seen in our lives."
Victim Julie Bridges, 42, a 13-year veteran of the county force, had been collecting supplies to keep helping civilians when she collided with Joseph Ossman, 53, who had been going to work.
"She worked the shelter all night and was going home to retrieve some more items and then go back to the shelter," Hardee County Sheriff Arnold Lanier told the Hardee County Herald-Advocate.
The wreck was reported at 6.53am (local time), having been found at the intersection of Old Crewsville Rd and SR 66 in Zolfo Springs. No other vehicles or people were involved.
The Florida Highway Patrol is investigating the incident, and has not yet ruled whether the winds and rain caused by Irma at the time of the crash directly influenced the accident.
Bridges was a mother of an eight-year-old boy and a member of the sheriff's Honor Guard. Ossman had been working at the Hardee Correctional Institute for 21 years.
"We are heartbroken by this loss, and our thoughts and prayers are with his family and fellow officers at this time," Corrections Secretary Julie Jones told the Miami Herald.
They were not the only people to die amid the deluge.
Another man was killed after tropical-storm-strength winds caused him to lose control of the truck he was driving through Monroe County, which contains Key West. He had been carrying a generator, local officials told ABC News.
And an elderly man died of natural causes while sheltering in a school in the city of Marathon on the Keys, Larry Kahn, an editor for FlKeysNews.com, said.
"He was staying in one of the classrooms," Khan explained. "Police came up, along with a couple of nurses who are here, actually, got everyone out of the room and sealed it off."
Those deaths come after Irma claimed at least 25 lives in the Caribbean as it swept over several countries, destroying entire islands.
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