The drone footage and photos circulating on social media show what appears to be the unthinkable: Mass graves on a New York City island as the city struggles in the throes of a pandemic.

But city officials say the shocking images only tell a partial story: Hart Island — located just off the coast of the Bronx — has been used for more than 150 years as a place to bury the city's unidentified or unclaimed dead, or those whose families can't pay for a burial.

In a series of tweets and later at a press conference, New York City Mayor Bill de Blasio acknowledged that the number of burials on the island has increased, but said the only people being buried there are those who have not been claimed by a family member or loved one.

"It's a sad topic. Imagine anyone who passes away and there's no one there to claim the body. This has been the truth for generations," de Blasio said.

The mayor stressed that even the most recent burials on the island are not necessarily always related to the pandemic.

"These are people who no one after a period of time has claimed them, and not just COVID victims, but victims of all diseases, all reasons for fatality," de Blasio said. "So because there's just been unfortunately more people passing away, including those who are not claimed by any family, that's what's been happening at Hart Island. But that's the only thing that's been happening at Hart Island."

New York City has had more than 92,000 confirmed cases of COVID-19 and more than 5,800 deaths, according to data from Johns Hopkins University. At a press briefing Friday, New York Gov. Andrew Cuomo said an additional 777 people in the state had died of the virus from the day before. It's the fourth day the death toll has climbed above 700, a huge proportion of those deaths have occurred in New York City.

Cuomo said he's not normally briefed on Hart Island and knows nothing about the burial of COVID-19 victims there. But many of the Hart Island burials, captured by drone Thursday, are likely related to COVID-19, given the sheer number of deaths in the city from the virus. Jason Kersten — a spokesperson for the city's Department of Correction, which oversees the burials — told Reuters that burials on the island have increased five-fold and that two new trenches have been added.

As de Blasio said, some of the burials happening now could also be for people who died of unrelated causes, and whose remains have been sitting in the city's morgues for days, or even weeks, unclaimed. The city's Office of Chief Medical Examiner will hold a body for 14 days during the pandemic, according to a memo obtained by CNN.

De Blasio recently told NY1 that the city's morgues have "the ability to deal with even a more challenging situation" in terms of deaths from the virus, which have already led to the addition of refrigerated trucks to hold the bodies of COVID-19 victims.

"If, god forbid, we ever have to get to the point of a temporary burial, it would be individual by individual so that families could reclaim their loved ones when the crisis is over. But we're nowhere near that now," de Blasio said.

De Blasio added that rumors that the city government was considering temporary burials in public parks were "totally false."

"If there was ever going to need to be a place for burial, it would be Hart Island, where burials are already done," de Blasio said, also emphasizing that inmates from Rikers Island, who typically carry out the burials at Hart Island, would not be involved in any COVID-19 related burials. The Hart Island burials captured on film on Thursday were done by contract laborers, according to the city.

The New York City Department of Correction records the burial location of each person buried at Hart Island and people with "close personal ties" to the deceased can normally schedule visits to the grave sites, though those visits have currently been suspended. More than one million people are buried there.

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