SAN JUAN, Puerto Rico --  Officially, Puerto Rico says 64 people were killed in Hurricane Maria-related deaths.

• Harvard study says official death toll may be off by thousands
• Study estimates 4,645 more people died between September and December 2017 than in 2016
• Independent review from George Washington University coming soon

A new study from Harvard University says the number of deaths is actually in the thousands.

The research was published in the New England Journal of Medicine.

Researchers surveyed some 3,299 randomly chosen households across the island to get an independent estimate of what it called "all-cause morality" after the hurricane, including information about displacement, infrastructure loss and causes of deaths. The researchers noted there was a high mortality rate after Hurricane Maria devastated the island in September 2017.

The researchers say the results showed a 62 percent increase in the mortality rate compared to the same time in 2016, equalling 4,645 more deaths between Sept. 20, 2017 and Dec. 31, 2017 than usual.

The study says this suggests the actual toll for deaths related to Hurricane Maria in Puerto Rico is more than 70 times the official estimate.

Among the problems that may have contributed to the deaths, researchers found 31 percent of the households surveyed had disruptions in medical services, and more than 14 percent were unable to get medications. Maria has also caused the longest blackout in U.S. history, leaving millions without power, including hospitals and nursing homes.

Researchers said they went with a household survey because a variety of factors could lead to patients not being confirmed as disaster-related deaths in a place where infrastructure is severely compromised.

"In Puerto Rico, every disaster-related death must be confirmed by the Institute of Forensic Sciences," the study said. "This requires that bodies be brought to San Juan or that a medical examiner travel to the local municipality to verify the death, often delaying the issuance of death certificates.

"Furthermore, although direct causes of death are easier to assign by medical examiners, indirect deaths resulting from worsening of chronic conditions or from delayed medical treatments may not be captured on death certificates. These difficulties pose substantial challenges for the accurate and timely estimation of official all-cause hurricane-related mortality," it added.

The official death toll in Puerto Rico related to Hurricane Maria has long been a controversial topic. Several previous studies have suggested the death toll may be off. In February, Puerto Rico's governor said a team from George Washington University would lead an independent study. The team was supposed to release its report in May, but requested more time. 

Information from the Associated Press was used in this report.