* About 70 killed by storm, including 27 in U.S.
* Quarter of all homes in Florida Keys believed destroyed
* Millions still without power; outages may last over a week
* Authorities working to restore basic services (Updates number of deaths at nursing home, adds new damage estimates)
By Andrew Innerarity and Ricardo Ortiz
HOLLYWOOD, Fla./SAN JUAN, P.R., Sept 13 (Reuters) - H urricane Irma claimed eight more lives, including six patients at a Florida nursing home that lost power, authorities said on Wednesday, as millions struggled without electricity and residents trickled back to the devastated Florida Keys.
Categorized as one of the most powerful Atlantic storms on record when it rampaged through the Caribbean, Irma has killed at least 70 people, officials said.
At least 27 people died in Florida and nearby U.S. states, and destruction was widespread in the Keys, where Irma made initial U.S. landfall on Sunday and became the second major hurricane to strike the mainland this season.
Some 4.3 million homes and businesses, or about 9 million people, were without power at midday Wednesday in Florida and nearby states.
Police opened a criminal investigation at the Rehabilitation Center of Hollywood Hills, where two elderly residents were found dead at the facility and three later died at a hospital, officials said.
Some residents were evacuated early Sunday morning and some woke up feeling sick at the center, which had been without air conditioning, Broward County Mayor Barbara Sharief told reporters on Wednesday.
"The building has been sealed off and we are conducting a criminal investigation inside," Hollywood Police Chief Thomas Sanchez told reporters on Wednesday.
Prolonged power outages remains a risk for the elderly through the region, Sharief said.
Florida Power & Light said it had provided power to some parts of the Hollywood nursing home but that the facility was not on a county top tier list for emergency power restoration.
Irma caused about $25 billion in insured losses, including $18 billion in the United States and $7 billion in the Caribbean, catastrophe modeler Karen Clark & Co estimated on Wednesday.
The Florida Keys were particularly hard hit, with federal officials saying that 25 percent of homes were destroyed and 65 percent suffered major damage when Irma barreled ashore on Sunday as a Category 4 hurricane with sustained winds of up to 130 miles per hour (215 km per hour).
Most residents had left by then and police have barred re-entry to most of the Keys to allow more time to restore electricity and medical service and bring water, food and fuel. Some 10,000 Keys residents stayed put when the storm hit and may ultimately need to be evacuated, according to officials.
"I don't have a house. I don't have a job. I have nothing," said Mercedes Lopez, 50, whose family fled north from the Keys town of Marathon on Friday and rode out the storm at an Orlando hotel, only to learn their home was destroyed, along with the gasoline station where Lopez worked.
Federal Emergency Management Agency administrator Brock Long said warned that life on the Keys would remain tough.
"Citizens are frustrated about not being able to get the support they need right now," Long told CNN. "That's exactly why we asked them to leave."
Three people died of carbon monoxide poisoning, including children aged 13 and 16, after a portable gas generator was left running inside an Orlando-area house, the Orange County Sheriff's office said on Wednesday. Four others from the house were hospitalized.
President Donald Trump is due to visit the region on Thursday.
Irma wreaked total devastation in parts of the Caribbean, where at least 43 people have died.
People who fled their homes in hard-hit islands including St. Martin and the U.S. Virgin Islands that were all but cut off from the world for days arrived in San Juan late Tuesday.
Michael Benson, 65, of St. John in the U.S. Virgin Islands, said he lost everything.
"My house, my business, both my vehicles, everything is gone," said Benson, who was stopping in San Juan before continuing to Boston to seek refuge with his wife's brother.
"But we have life. We rode out that horrible storm in a shower that I had reinforced after Hurricane Marilyn," Benson added. "I told the man (who installed the shower), I told him, 'If the hurricane takes the rest of my house, I want this shower sticking up out of that slab like the last tooth in the mouth of a bum. And sure enough that's what's left."
Florida's largest utility, Florida Power & Light Co, said western parts of Florida might be without electricity until Sept. 22.
Irma hit the United States about two weeks after Hurricane Harvey plowed into Houston, killing about 60 and causing some $180 billion in damage, mostly from flooding.
Additional reporting by Daniel Trotta in Orlando, Florida, Bernie Woodall and Ben Gruber in Miami; Dan Whitcomb in Los Angeles, Letitia Stein in Detroit, Colleen Jenkins in Winston-Salem, North Carolina, Brendan O'Brien in Milwaukee and Gina Cherelus, Peter Szekeley and Scott DiSavino in New York; Writing by Scott Malone; Editing by John Stonestreet and Jeffrey Benkoe