There's another question you have to ask first: Is 500 deaths over 35 years an outrageous number, one that is significantly greater than the average across the country?
I don't know what the average number of in-prison deaths is for the United States. But I can do some math and 500 deaths over 35 years equals 14.28 deaths a year. Is that a crazy number? Doesn't seem like it - 14 is a small number. But this is a small area in the country and it equates to at least 1 death a month for 35 years.
When thought through this way it seems like an awfully erie trend...
Certainly the overall number is not super high.
A story from Florida last year
http://www.vocativ.com/underworld/crime/prison-deathThere is no "authority" in the United States that has had real credibility investigating that sort of thing. The FBI will investigate local police sometimes, if there is publicity, but they don't investigate themselves very well,
http://kennethtrentadue.com/ comes to mind.
In most European countries there is not even a suspicion that prisoners are occasionally killed and the deaths covered up as suicide or 'natural'. But in some countries, like the United States, it is an issue that probably should get more attention.