Between 1,006 and 1,272 people died in Puerto Rico as a result of Hurricane Maria, a number that is 15 to 20 times higher than the official government estimate, according to a new research letter released in the Journal of the American Medical Association on Thursday.

"They clearly were not counting all of [the deaths]," Jeffrey T. Howard, assistant professor of health at the University of Texas at San Antonio, and one of the study's authors, told CNN. The Puerto Rican government has estimated that 64 people died because of the storm.

"They can't be. There were just too many deaths."

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Howard and Alexis Santos-Lozada, from Pennsylvania State University, another author of the study, produced their estimate by comparing the number of deaths in the months following the hurricane to those in previous years, according to CNN.

The researchers estimated that 459 excess deaths occurred in September, which was the month of the storm. They also estimate that 564 excess deaths occurred in October and that 116 happened in November. They say that the mortality "had returned to a level within historical variation" in December, according to CNN.

The network also notes that their midpoint estimate is that 1,139 "excess deaths" were related to the storm.

The study comes as the Puerto Rican government faces increasing scrutiny from lawmakers and academics over how many people died because of Hurricane Maria, which ravaged Puerto Rico and other Caribbean islands in September.

Earlier this year, a Harvard University study published in the New England Journal of Medicine estimated that the death toll from the hurricane and its immediate aftermath was 4,645 people — 70 times higher than the official government estimate.

The Harvard study, which used household surveys, noted that Puerto Rican officials have declined to release mortality statistics that could shed more light on the lives lost after Maria. Puerto Rico Gov. Ricardo Rosselló said earlier this year that there would be "hell to pay" if officials continued to decline to release data.

Puerto Rico's Institute of Statistics then filed a lawsuit seeking updated information on the death toll. The government responded by releasing data that indicated there were at least 1,400 more deaths in the months after Hurricane Maria than during the same period the year before, according to The Washington Post.

CNN notes that Rosselló previously announced that Puerto Rico had hired George Washington University researchers to examine the death toll. The university has said the initial report will be released this summer.