Death row convicts can't rely on an eleventh-hour appeal, although some like Richard Glossip secured a fourth stay of execution in 2015 when the legalities of execution came into question. Glossip, convicted in 1997 for murder, maintains his innocence, highlighting the possibility of states executing the wrongfully convicted. Controversial executions make up a large number of death penalty cases in the United States, in part due to new forensic science capabilities. With capital punishment comes the burden to make sure guilt is proven beyond a reasonable doubt. US executions must be consistent with the Bill of Rights and established case law. Minors or those with diminished mental capacity can't be executed, and the punishment can't be cruel or unusual. This means the humane punishment - no torture or degradation - has to match the severity of the capital crime.
Unfortunately, these rules have been broken quite often in death penalty cases. Especially in the early 20th century, minority suspects were often given incompetent defenses with trials presided over by hostile judges and juries. Children have been executed, along with suspects whose mental capacity left them unfit to stand trial. And many people have gone to their deaths proclaiming their innocence - only to be proven right decades later.
Here are some of the most controversial executions and death penalty cases in US history.
Sacco and Vanzetti
Italian immigrants Nicola Sacco and Bartolomeo Vanzetti were executed in 1927 after a highly contested series of trials over the shooting death of a security guard and paymaster during a 1920 armed robbery.
The trials are now seen to be highly flawed, featuring a poor defense, conflicting witnesses speaking in thick Italian accents, and evidence of the jury tampering with evidence. The men were followers of Italian anarchist Luigi Galleani, and anti-Italian sentiment almost certainly played a role in their execution.
After their convictions, the accused men waged a six-year legal battle that went all the way to the Supreme Court twice, with numerous public figures calling for new trials. Another man even confessed to the shootings, ex-convict Celestino Madeiros, but the die was cast, and the two men died in the electric chair on August 23, 1927. Historians still debate whether either man really pulled the trigger, although dubious forensic evidence later indicated that Sacco's gun was used in the killing.
In 1944, Stinney was found guilty for the alleged rape and murder of two pre-teen girls. The only "evidence" against Stinney was that he'd spoken with the two girls at some point before their murder. An all-white jury deliberated for 10 minutes, after a trial that took just a day. At 14, Stinney was the youngest person executed in the US in the 20th century - with less than three months passing between the murders and his execution.
In 2013, a local historian persuaded a South Carolina circuit court to reopen the case. Stinney was given a new trial and exonerated because the boy's defense attorney had taken no action on his behalf, and Stinney had been beaten into confessing.
see more on George Stinney
The Scottsboro Boys
Even 80 years later, the phrase "Scottsboro Boys" is shorthand for racially-based death sentencing.
Based on the judgment of all-white juries, nine black teenage boys were found guilty of raping two white women on a freight train in 1931. Eight were sentenced to death via the electric chair. The ninth boy, at only 13 years old, was sentenced to life in prison. The proceedings were a farce from the beginning, with all nine trials being handled in just one day, last-minute defense attorneys, and a lynch mob demanding the surrender of the teenagers outside the court.
The convictions led to demonstrations around the country, and the case eventually made it to the Supreme Court. The convictions were reversed because of the lack of an adequate defense, with charges being dropped against four of the men. After numerous appeals, retrials, and reversals, none of the men were executed, though many served long prison terms - all based on conflicting evidence.
Chessman became a key figure in anti-death penalty sentiment in the 1950s after being convicted of robbery, kidnapping, and rape in 1948 under the pseudonym the "Red Light Bandit." The jury determined that Chessman had caused bodily harm during one of the alleged kidnappings, making him eligible for death - despite none of the victims actually dying.
From death row, Chessman wrote books maintaining his innocence and insisting that his original confession had been coerced through torture. His trial became a media circus, with Chessman defending himself and a botched transcript that was noticeably edited after the death of the court stenographer. There was widespread outrage over the case, and Chessman had numerous public figures clamoring for a new trial, including former first lady Eleanor Roosevelt, writer Ray Bradbury, and poet Robert Frost.
On May 2, 1960, Chessman finally went to the gas chamber at San Quentin Prison - just as a legal secretary called with news that a federal judge had issued one more stay of execution. But by the time the news reached prison officials, Chessman was dead.see more on Caryl Chessman