Where are the Hart Island burial records located?

I think a friend of mine was buried on Hart Island. How do I find out if this is true?

I believe my relative is buried on Hart Island. How do I find out for sure?

On the death certificate nothing is listed under cemetery. Does this mean that my relative is buried on Hart Island?

I am a graduate student or a researcher. How can I visit Hart Island?

What if I don't know exactly when a person died?