A team of researchers spent six months poring over death certificates, speaking to funeral home directors and interviewing doctors to arrive at a death toll of nearly 3,000 people as result of Hurricane Maria's devastation to Puerto Rico, according to the lead author of the study.
"This was a scientific research project commissioned by the government of Puerto Rico and independently done," said Carlos Santos-Burgoa, who led the study from George Washington University's Milken Institute of School of Public Health.
Santos-Burgoa pushed back on President Donald Trump's tweets Thursday questioning the study's estimate of the death toll. Without offering evidence, Trump said Democrats were involved in an effort to overstate the results in order to discredit him.
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"We stand by what we did," Santos-Burgoa told USA TODAY in a telephone interview, adding that the study that found 2,975 deaths underwent rigorous peer review.
Trump's comments drew sharp criticism, both on and off the island of Puerto Rico.
Puerto Rico has adopted the GWU study's estimate as its official death toll. In a statement, the university defended its research.
"Our results show that Hurricane Maria was a very deadly storm, one that affected the entire island but hit the poor and the elderly the hardest," the GWU statement said. "We are confident that the number – 2,975 – is the most accurate and unbiased estimate of excess mortality to date."
The analysis, released in August, suggested that Hurricane Maria was the second-deadliest storm to ever hit U.S. shores. The deadliest hurricane was 1900 in Galveston, Texas. That storm killed an estimated 6,000 people.
"The victims of Puerto Rico, and the people of Puerto Rico in general, don't deserve to have their pain questioned," Gov. Ricardo Rosselló said in a Facebook video Thursday. "We accept the number of deaths ... We left this analysis in the hands of scientists and experts."
Several prominent Republicans distanced themselves from Trump's comments on the Puerto Rico death toll, including House Speaker Paul Ryan.
Preparations for the study began last December when Puerto Rican officials approached the Milken Institute. At that time, the official toll was still 64 – a low number that would remain the official count for months.
By February, Santos-Burgoa's team had a contract with the government to conduct the study. A key provision was that the study be conducted independently without input from Puerto Rican – or any other – government officials, he said.
"They never made any suggestions of anything," Santos-Burgoa said.
A team of 11 investigators from GWU, four researchers from the University of Puerto Rico and other local community leaders spent months poring over death certificates on the island, looking at vital statistics data stretching back seven years before the storm to compare mortality rates.
The GWU researchers included epidemiologists, a demographer, a public health nutritionist, environmental health scientists, two public health research assistants, an anthropologist, a behavioral scientist and two health communication experts.
They interviewed doctors, health officials, funeral home directors, hospital directors, forensic pathologists and others involved with the death certification process to get a full picture of how deaths were classified after the storm and used guidelines set by the U.S. Centers for Disease Control and Prevention.
Along the way, their methodology was reviewed by a Johns Hopkins University researcher and by a panel of national and international experts in the field, Santos-Burgoa said.
He said the team was "obsessed" with getting as much peer input as possible.
"We were doing the project, but we had other eyes looking at us," Santos-Burgoa said.
The report concluded that doctors and forensic pathologists on the ground in Puerto Rico didn't have the correct guidance – from either the state or federal government – to accurately classify deaths, leading to the low figure.
Besides bringing closure to thousands of Puerto Ricans across the island, the study's estimate is also necessary to determine what went wrong and prevent widespread casualties during future storms, Santos-Burgoa said.
He said his team is trying to get funding for another study looking at the main causes of death during Maria, something that could help prevent more deaths in future storms, he said.
"We have to understand how these deaths happened so we can prevent them from happening again," Santos-Burgoa said.