The hurricane that struck Puerto Rico in September was responsible for more deaths than the Sept. 11 attacks and Hurricane Katrina combined, according to an analysis published on Tuesday in the New England Journal of Medicine.

The group estimates that 4,645 people probably died after Hurricane Maria struck the island last September. That’s 70 times higher than the official death toll of 64 people reported by government officials.

By comparison, the Sept. 11, 2001, terrorist attacks killed 2,977 people and the deadliest hurricane in U.S. history, Katrina, killed 1,833 in 2005.

A group of 15 researchers, with most based at Harvard’s T.H. Chan School of Public Health, used a statistical method that hadn’t been used for measuring deaths from Hurricane Maria. Working with Carlos Albizu University and Ponce Health Sciences University of Puerto Rico, the team surveyed 3,299 households and 9,522 people between January and February 2018, a few months after Maria made landfall and devastated the country.

What the researchers conducted was akin to a census study, asking households to provide data about migration within the household: Who had lived there? Who had moved in and out? Most importantly, who had died?

The group’s calculations landed them on the 4,645 figure, from deaths that could not have occurred had the island not been plunged into a humanitarian disaster and received no medical aid in the critical days following Hurricane Maria’s landfall.

They found that about one-third of the hurricane fatality rate could be contributed to delayed medical care, and that the median age of the people who stayed home and/or died was about 50 (compared to the more 20-something demographic that escaped the island). The more remote a person was, the less likely a household would have access to electricity, water, or phone coverage; an astounding 83 percent of households lacked electricity even up to December. (The Daily Beast reached out to the lead researchers for comment and did not receive a response.)

Which meant that the perfect storm of Maria caused a perfect on-the-ground storm for mortality, with researchers tracking a spike in deaths of 62 percent in the months after the storm.

This isn't the first time that people have questioned the death toll from Hurricane Maria. Public health officials and those on the ground in Puerto Rico have long questioned the official death count of 64, which seemed incredibly, artificially low. Various agencies have said for months that the death toll was almost certainly higher. In December, a New York Times analysis put the death toll at 1,052. Puerto Rican demographer Alexis Santos predicted a similar number in a paper in November with Soc ArXiv. CNN surveyed funeral homes in the aftermath of the hurricane and said the number was at least in the 500s.

The Harvard study, however, not only backs up those initial analyses with statistical evidence but suggests that the death toll was even worse than we’d initially imagined.

San Juan Mayor Carmen Yulín Cruz Soto, who skyrocketed to prominence for her fiery speeches against President Donald Trump in the aftermath of the hurricane, has said she was also skeptical about the official figure put forth by the American government.

“It took too long to understand the need for an appropriate response was NOT about politics but about saving lives,” the mayor pointed out.

“Harvard points out what I have been denouncing since October 2017,” she added. “There are many deaths caused by poor crisis management.”

Carlos Mercader, the executive director of the Puerto Rico Federal Affairs Administration welcomed the study and admitted that the death figure provided by the government is not accurate.

“ Harvard points out what I have been denouncing since October 2017. There are many deaths caused by poor crisis management. ” — San Juan Mayor Carmen Yulín Cruz

“The Government of Puerto Rico welcomes the newly released Harvard University survey and we look forward to analyzing it,” he said in a statement. “As the world knows, the magnitude of this tragic disaster caused by Hurricane Maria resulted in many fatalities. We have always expected the number to be higher than what was previously reported.”

Rossello, the governor of Puerto Rico, was even more receptive to the study and didn’t seem surprised with its results, pointing out that back then the island had an outdated protocol to account for the deaths.

“We know that the accurate death toll is higher,” he said. In February, Rosselló assigned a team of research from Milken Institute School of Public Health at the George Washington University to revise the death count related to the hurricane but the researchers requested additional time to complete the study.

The Daily Beast has reached out to George Washington University on numerous occasions, but according to Kathleen Fackelmann, director of Media Relations, the research team will make no comment until the study is finished.

“The study is ongoing and the research team has no results yet. Because of the intensive nature of the research and the short timeframe I do not have anyone who can take time out for interviews,” said Fackelmann.

Mercader said that the Harvard study is a first step towards establishing a more honest narrative of what happened in Puerto Rico, shining a light as to what happened in the deadly aftermath of a storm that so pummeled the island, it is still recovering from it. “Both studies will help us better prepare for future natural disasters and prevent lives from being lost,” said Mercader.

Of course, critics might poke through the argument here and say that the estimated number is based on a survey of randomly chosen households, that it doesn't necessarily mirror what might be the actual death toll in the area.

Some, including Puerto Rico’s secretary of the Department of Public Safety, have questioned the legitimacy of the study, arguing that the result was based on hearsay and not real data, since the study did not have cooperation from the Puerto Rican government..

“The study was conducted based on surveys and did not use our database. A survey is not the same as using real data,” said Hector Pesquera, secretary of Puerto Rico’s department of public safety.

However, the researchers disagree in the paper. While an estimate, the number can be seen as pretty close to the actual number, they argue, emphasizing that they stratify the population, which means that households were represented across the country’s barrios, socioeconomic classes, and “remoteness” measures to ensure accuracy.

Not only that, but before and after estimates of mortality remained conspicuously high after the hurricane had passed, not matching normal mortality rates for that time of year, suggesting an event had caused the spike—and that event had to be Hurricane Maria. And that doesn’t even begin to count those who might have died alone.

The hurricane mortality patterns also matched up well with what Americans have observed in the days after a hurricane on the mainland, with interruption of medical care a huge contributor to mortality rates. That basic infrastructure remained out of use even months after Maria’s touchdown mean that those on the brink of death, particularly in remote regions, lost nearly all hope of being rescued and given urgent medical care.

The Harvard study, importantly, did not have the cooperation of the territory’s government in producing its numbers. The GWU study, which was scheduled to come out this month but whose directors have been so overwhelmed with data that they asked for an extension, does have the government’s cooperation. That means that the estimate the authors put forth will be almost immediately either defended or shattered.

—with additional reporting by Pablo Venes in Puerto Rico