New York is unique among American cities in the way it disposes of the dead it considers unclaimed: interment on a lonely island, off-limits to the public, by a crew of inmates. Buried by the score in wide, deep pits, the Hart Island dead seem to vanish — and so does any explanation for how they came to be there.
The use of mass graves in these cases is not new.
"Unearthing the Secrets of New York’s Mass Graves - The New York Times" https://www.nytimes.com/interactive/2016/05/15/nyregion/new-york-mass-graves-hart-island.html