People gather opposite Downing Street during a protest against the execution of a young woman in Iran, in October of last year. Amnesty International says that Iran has undergone an "unprecedented spike" in executions in recent months. Graham Mitchell/Barcroft Media/Landov hide caption

itoggle caption Graham Mitchell/Barcroft Media/Landov

An undated file photo from the Arizona Department of Corrections shows inmate Joseph Rudolph Wood, who was executed Wednesday. After the lethal injection process began, Wood reportedly remained alive for nearly two hours. AP hide caption

itoggle caption AP

Joseph Paul Franklin in a photo taken in 2012 by the Missouri Department of Corrections. /Reuters /Landov hide caption

itoggle caption /Reuters /Landov