These 7 People Were Pardoned AFTER They Were Executed

As many as 300 U.S. inmates sentenced to death over a 30-year period were probably innocent, according to a recent study in the Proceedings of the National Academy of Sciences.

The courts converted many of their death sentences to life, the study found. The U.S. judicial system, however, has actually killed innocent people.

While doubt exists in many past executions, seven people have received official pardons after their executions in prison. Read their stories below.

Screenshot via 9NewsJoe Arridy, executed in 1939

Joe Arridy

Executed for the rape and murder of a 15-year-old Colorado girl, Joe Arridy died by lethal gas in 1939. As his last request, the 23-year-old asked for lots of ice cream and his toy train, the Denver Westward reported. With an IQ of 46, Arridy couldn't quite grasp the concept of death.

Arridy confessed to attacking Dorothy Drain and her 12-year-old sister, Barbara, with a hatchet in 1936. (Barbara survived the attack.) Decades later, overwhelming evidence proved his innocence. He wasn't in town at the time of the killing, and someone else even admitted to the crime.

Police coerced Arridy's confession, the state said when it finally pardoned him. Aside from the fact that he was innocent, imposing the death penalty on someone as intellectually disabled as Arridy would also be considered unconstitutional today.

Colorado Gov. Bill Ritter officially pardoned Arridy in 2011. "Pardoning Mr. Arridy cannot undo this tragic event in Colorado history," Ritter said. "It is in the interest of justice and simple decency, however, to restore his good name."

Thomas Griffin and Meeks Griffin

Thomas and Meeks Griffin, relatively wealthy black farmers, were electrocuted in 1915 for killing a 73-year-old white Civil War veteran named John Q. Lewis.

A prominent legal historian's recent research, however, revealed Lewis was having an affair with a much younger black woman, CNN reported. That woman and her husband may have actually committed the murder.

One hundred years after South Carolina executed the Griffin brothers, the state issued its first posthumous pardon in a unanimous vote. The Griffins' grandnephew, nationally syndicated radio host Tom Joyner, spurred research into the case after Harvard professor Henry Louis Gates Jr. traced his family history and discovered the double-execution.

"It's good for the community. It's good for the nation. Anytime that you can repair racism in this country is a step forward," Joyner told CNN.

AP Photo/Georgia Department of Corrections, FileLena Baker, the only woman Georgia ever executed

Lena Baker

The only woman executed in Georgia, a 44-year-old black maid named Lena Baker, died in the electric chair in 1945 for killing her employer, Ernest Knight.

At her trial, Baker told the all-white, all-male jury Knight had imprisoned her and threatened to shoot her if she tried to leave, The Guardian reported. She finally grabbed a gun and shot him when he held up a metal bar to strike her, she testified.

Baker and Knight had a sexual relationship that sparked outrage in the community, and his son had threatened her and beat her multiple times.

Sixty years after her death in 2005, the Georgia Board of Pardons and Paroles granted her family an official proclamation, pardoning her. While the board didn't find her not guilty, it called the decision to refuse her clemency a "grievous error," according to The New York Times.

John Snowden

The murder of a pregnant naval officer's wife in Maryland sent John Snowden, a 29-year-old black man, to the gallows in February of 1919. He professed his innocence until the day he hanged. Even given one last chance to confess, he said, "I could not leave this world with a lie in my mouth."

Snowden had a loose connection to the victim, Lottie Brandon: He drove an ice truck in her neighborhood. Yet he endured hours of questioning and abuse from the police.

Two of the main trial witnesses eventually recanted their testimony, according to a report distributed through the Death Penalty Information Center. The jurors who convicted him even became convinced of his innocence: 11 of the 12 wrote the state government, asking it to commute Snowden's sentence.

In 2001, then-Maryland Gov. Parris Glendening finally gave John Snowden a full pardon.

"The more I looked into it, the more I said, 'Something's just not right here,'" Glendening told the Baltimore Sun.

Nebraska State Historical SocietyWilliam Jackson Marion at the gallows

William Jackson Marion

Four years after William Jackson Marion was hanged in 1887 for supposedly killing his railroad co-worker, James Cameron, the "murdered" man turned up alive and well.

Days before Cameron disappeared, he and Marion made a business agreement that clearly benefitted Marion. Cameron's mother also told police she suspected Marion's involvement in her son's disappearance.

About 11 years later, a dead body, wearing clothes similar to Cameron's, was found on a Native American reservation. After a mistrial, a jury eventually found Cameron guilty of the murder and sent him to the gallows.

Cameron - who showed up alive and well four years after the execution - explained to authorities he'd fled for Mexico to avoid a shotgun wedding in Kansas. On the centennial anniversary of Marion's execution in 1987, Nebraska Governor Bob Kerrey granted Marion a full posthumous pardon.

Commonwealth of PennsylvaniaJack Kehoe's death warrant, signed by the governor

Jack Kehoe

Jack Kehoe - leader of an anti-Civil War group called the Molly Maguires - was hanged by the state of Pennsylvania in 1878 for the murder of a dissenting coal miner named Frank Langdon.

Police first charged Kehoe's group with threatening to kill Langdon, and when he turned up dead days later, they handcuffed Kehoe first.

Despite no physical evidence linking Kehoe to the scene of the crime, a mining big-wig at the time named Franklin Gowan rallied for Kehoe to be put to death.

In 1978, Kehoe's great-grandson asked for his pardon. He claimed Gowan stacked the jury, some of whom didn't even speak English, against Kehoe, among other stilted circumstances, the Beaver County Times reported.

In 1979, then-Gov. Milton J. Shapp issued a full pardon to Kehoe, proclaiming him innocent.