Alan Greenblatt

States have always struggled to find humane ways to carry out the death penalty. For a generation, they have favored lethal injection, but that method has become increasingly problematic.

It's coming under increased scrutiny following the death of Clayton Lockett, who died Tuesday of a heart attack after writhing visibly during an execution attempt in Oklahoma.

The execution "fell short" of humane standards, White House spokesman Jay Carney told reporters on Wednesday.