That is a stunning increase over the 64 deaths counted by Puerto Rico Gov. Ricardo Rossello's administration.
"The lessons learned from this report and subsequent studies will help not just Puerto Rico, but other regions in the USA and around the world that face the ongoing threat of hurricanes and other natural disasters", said Lynn R. Goldman, MD, MS, MPH, Michael and Lori Milken Dean of the GW Milken Institute SPH and a co-author of the report.
Some media and academic studies estimated the death toll at more than 1,000, and a government report to Congress conceded that there may have been 1,400 more deaths in Puerto Rico after the storm than the previous year.
"The American people, including those grieving the loss of a loved one, deserve no less", Sanders said in a statement.
Speaking at a news conference in Puerto Rico's capital, San Juan, Rossello said his government was adopting the findings as the official account of human life lost in the disaster, "even though it is an estimate". They are therefore unaware of "appropriate death certification practices, especially in a disaster setting", the report found.
"That caused a number of issues", said Lynn Goldman, dean of the Milken institute.
The Puerto Rican government said the storm caused 64 deaths. "It's fairly striking that you have so many households without electricity for so long".
Based on the study's findings, Maria's fatalities surpass those of Hurricane Katrina, which killed about 1,800 when it hit the Gulf Coast in 2005, and every other USA natural disaster since the early 20th century.
To arrive at the 2,975 figure, the study looked at historical death patterns from 2010 to 2017 to estimate how many people would have died had Hurricane Maria not hit the island. "Now, almost a year later, we are pleased to be coming close to the end of our role in the reconstruction of Puerto Rico's transportation infrastructure", Acrow CEO Bill Killeen said.
The researchers found that the risk of death was 45 percent higher for those living in impoverished communities, and that men older than 65 saw a continuous elevated risk of death.
The team found that lack of communication, well established guidelines and lack of training for physicians on how to certify deaths in disasters, resulted in a limited number of deaths being identified as hurricane related.
It analyzed death certificates and other mortality data for six months from September 2017 through February 2018.
"Overall, we estimate that 40% of municipalities experienced significantly higher mortality in the study period than in the comparable period of the previous two years", the report says.
Researchers hope to continue their research, interviewing families of those who died to better understand how the hurricane impacted their deaths.
"Others expressed reluctance to relate deaths to hurricanes due to concern about the subjectivity of this determination and about liability", the report said. Maria was a Category 4 with 154-mph winds.
She added that she worries whether Puerto Rico, which is trying to restructure a portion of its more than $70 billion public debt amid a 12-year recession, can adopt any of the recommendations.