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Hurricane Maria ravaged Puerto Rico almost a year ago, but scientists are still trying to unpack the many layers of damage it inflicted on the island.

Researchers are becoming more certain, though, of a sobering reality: Disaster deaths can last for far longer than we suspect.

A new study by a group of researchers from the Harvard T.H. Chan School of Public Health and other institutions suggests that the number of deaths caused by Maria in a three-month period (from impact on September 20 until that December) could figure around 4,600 people. Perhaps more importantly, this estimate (if accurate) would mean the mortality rate during this window of time was 62% higher than the mortality rate during the same period the year before.

This unexpectedly large number of what scientists are calling “excess deaths” surpasses the number of people who died in Hurricane Katrina (1,833) and the September 11, 2001 terror attacks (2,996). Meanwhile, the government’s official casualty count from Hurricane Maria remains comparatively low (64).

The process of determining fatalities is a messy one, plagued with uncertainties. Much of the time, people struck by a catastrophe like Hurricane Maria die alone—maybe even forgotten. In other cases, they survive the initial disaster but die weeks or months later due to lack of medical supplies, food, or water.

In Puerto Rico, disaster-related deaths have to be confirmed by the Institute of Forensic Sciences, which means bodies need to be brought to San Juan or examiners themselves have to make the trek over to the deceased. Because of this, officials and researchers will often turn to door-to-door sampling of communities in order to get a sense of how many people were lost to a specific weather event.

For this particular analysis, researchers visited more than 3,000 residences across Puerto Rico and spoke with people who were still living there. They wanted to make their interviews as unbiased as possible, so they asked simple questions and let occupants report their difficulties from their own perspective. Collectively, these residents reported 38 deaths amongst their households. The team of scientists then converted that number into a mortality rate and then extrapolated the data to the overall population.