Hurricane Maria killed more than 4,600 people — more than 70 times the official toll of 64, study says.

John Bacon | USA TODAY

Show Caption Hide Caption Six months after Hurricane Maria, schools are a refuge for students José Campeche High School in San Lorenzo, Puerto Rico still hasn’t seen any recovery money from state or federal agencies for repairs, said Candido Rivera, the school’s director. “Everything we’ve done, we’ve done by ourselves,” he said.

Hurricane Maria likely killed thousands of people across Puerto Rico last year, more than 70 times the official estimate, a Harvard study released Tuesday says.

Authorities in Puerto Rico placed the death toll at 64 after Maria roared through the island Sept. 20, destroying buildings and knocking out power to virtually the entire U.S. territory of more than 3 million people.

Researchers at the Harvard T.H. Chan School of Public Health, however, surveyed more than 3,000 households on the battered island. By extrapolating those findings, researchers determined that at least 4,645 "excess deaths" occurred during the storm and the weeks that followed.

The researchers said the number was conservative and that the death toll likely exceeded 5,000. Many of the deaths were due in part to power outages that crippled medical and other services.

The study was published in the New England Journal of Medicine.

"Our results indicate that the official death count of 64 is a substantial underestimate of the true burden of mortality after Hurricane Maria," the study says.

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Rep. Bennie Thompson, D-Miss., who serves on the House Committee on Homeland Security, issued a statement calling the Harvard totals "heartbreaking" and describing the federal response to the storm as "woefully inadequate."

"The 2018 hurricane season begins this week, and it is critical that we do not repeat the mistakes of last year," Thompson said. "We can and must do better for all Americans.”

Only one hurricane to hit the U.S. has taken more lives than the study estimates Maria did. The Great Galveston Hurricane of 1900 killed more than 6,000 people.

More than 1,800 people died when Katrina roared across the U.S. Gulf Coast as a Category 5 hurricane in 2005. More than 1,500 of those deaths were in Louisiana.

The Harvard study notes that in Puerto Rico every disaster-related death must be confirmed by the Institute of Forensic Sciences. The system counts only bodies that are brought to San Juan or were confirmed by a medical examiner traveling to the local municipality.

The system also fails to capture indirect deaths resulting from worsening of chronic conditions or from delayed medical treatments, the study says.

"These difficulties pose substantial challenges for the accurate and timely estimation of official all-cause hurricane-related mortality," the report says.

Maria raked across Puerto Rico as a Category 4 hurricane, the strongest storm to hit the island in 89 years and among the strongest ever to strike the U.S.

Maria was one of three horrific hurricanes in 2017 — Harvey and Irma were the others. All three are among the costliest hurricanes in U.S. history, according to the National Oceanic and Atmospheric Administration says.

Maria caused an estimated $90 billion in damage, making it the third costliest ever in the United States. Katrina remains the costliest hurricane on record at $160 billion.

The Federal Emergency Management Agency, or FEMA, has approved $1 billion in individual assistance grants for residents of Puerto Rico. More than $500 million has been designated for repairs to bridges, government buildings and other structures.