31 US states, the federal government and the US military use the death penalty. However, I've seen in news reports that different convicts are executed in different ways. The usual method seems to be through lethal injection, but I have also seen reports of convicts being executed through firing squad or being hanged.
Who decides in which way capital punishment should be delivered in the US? And is there some kind of oversight which prevents states from carrying it out with especially cruel and humiliating methods?