(Reuters) - The official estimate of Puerto Rico's death toll from Hurricane Maria, the most powerful storm to strike the Caribbean U.S. island territory in nearly 90 years, has been raised to 2,975, up from 64, Governor Ricardo Rossello said on Tuesday.

The updated figure came from an academic study commissioned by Rossello from George Washington University's Milken Institute School of Public health.

It calculated the number of deaths that could be attributed directly or indirectly to Maria from the time it struck in September 2017 to mid-February of this year, based on comparisons between predicted mortality under normal circumstances and deaths documented after the storm.

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(Reporting by Steve Gorman in Los Angeles and Gabriella Borter in New York, Editing by Rosalba O'Brien)