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Nearly 5,000 people died in Hurricane Maria’s aftermath in Puerto Rico, far greater than the 64 deaths counted as the storms’ official toll, according to a new study from Harvard.

The findings, published Tuesday in the New England Journal of Medicine, show  there were 4,645 “excess deaths” between Sept. 20, 2017, when the storm made landfall on the island, and Dec. 31, 2017. Researchers found the official death toll is a “substantial underestimate of the true burden of mortality after Hurricane Maria.”

Many of the deaths came after the storm itself as residents struggled without potable water and electricity.

The official death toll has been questioned for months.

When President Donald Trump visited Puerto Rico on Oct. 3, he noted the storm’s relatively low death toll — then 16. By the end of the day, the official count had more than doubled.

A CNN survey of half the island’s funeral homes found about 500 Hurricane Maria-related deaths. That number was about nine times the official death toll. The number of deaths calculated in the new study is about 70 times the official count.

Researchers reached the number by doing door-to-door interviews in Puerto Rico. They talked to residents in 3,299 households. They used the responses to determine there were 14.3 deaths per 1,000 Puerto Ricans from the date that Hurricane Maria hit through the end of the year. They applied the rate to the entire country to find how many “excess deaths” there were.

The study notes that its total of 4,465 deaths is likely an underestimate.

The 2018 Atlantic hurricane season begins Friday and many are concerned that Puerto Rico is not prepared for additional storms. The administrator of the U.S. Federal Emergency Management Agency recently warned the island was not ready for another disaster and that they were “running out of time.