Over 4,600 people died in Puerto Rico during the aftermath of Hurricane Maria more than 70 times the official death toll according to a new Harvard study released Tuesday.

The study, published in the New England Journal of Medicine , found Hurricane Maria left at least 4,645 people dead in Puerto Rico after the storm made landfall on the island on Sept. 20, 2017. This figure stands in stark contrast with the Puerto Rican governments official tally of 64.

The Harvard researchers conducted a survey of 3,299 randomly chosen households across Puerto Rico, in which participants were asked about displacement, infrastructure loss and causes of death.

Interruption of medical care was the primary cause of the increase in mortality rates in the months after the hurricane, according to the study. Researchers said their 4,645 estimate is likely to be conservative and the death toll may actually exceed 5,000.