In 1931, an all-white jury took just four hours to convict Alexander McClay Williams, a Black teenager, in the stabbing death of a matron at the Glen Mills School for Boys in Delaware County.

Five months later, Williams, 16, was executed, becoming the youngest person in Pennsylvania history to be put to death. His family spent decades trying to prove his innocence, and this week — with help from the great-grandson of the lawyer who represented him at trial — Williams was posthumously vindicated.

A Delaware County judge overturned his conviction for a crime prosecutors now say he did not commit.

The ruling, handed down Monday in the same courtroom where Williams had been convicted 91 years ago, was met with thunderous applause from his relatives, including his only surviving sibling, Susie Williams-Carter.

“I’m just happy that it finally turned out the way it should have in the beginning,” Williams-Carter, 92, told The Inquirer. “We just wanted it overturned, because we knew he was innocent, and now we want everyone else to know it, too.”

For Sam Lemon, it was a victory that also honored his great-grandfather, William Ridley, the county’s first Black attorney, who was given just $10 and a few weeks to make a case that Williams did not kill Vida Robare, a white matron at Glen Mills who was found dead inside a cabin at the school. She had been stabbed 47 times with an ice pick, two of her ribs were broken, and her eye was fractured.

Robare’s ex-husband, who had a history of domestic violence against her, called police to report finding her dead. He was the last person to be seen with her, court records show, but was quickly ruled out as the killer. Suspicion fell on Williams, a teen who had been sent to Glen Mills after committing crimes that included setting a fire that destroyed a barn and caused $25,000 in damage, and burglarizing a post office at the age of 12.

Judge W. Roger Fronefield had sentenced Williams to an indeterminate stay at Glen Mills, a facility that Lemon said was little more than a jail for children at the time. Four years later, the same judge would sentence Williams to death in Robare’s murder.

Lemon, now 70, spent many childhood summer afternoons with his grandmother, listening to stories about her father, Ridley, the lawyer. He became fascinated by Williams’ case, a rare and troubling loss in his great-grandfather’s legal career.

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So about 30 years ago, Lemon, a college administrator who lives in Media, began digging into the case and discovered evidence that convinced him Williams was not guilty. For one thing, he said, prosecutors failed to introduce key evidence at trial, including a man’s bloody handprint that was found at the scene. And Williams had been busy working elsewhere on the grounds at the time of Robare’s death. To have committed the crime, Lemon said, the teen would have needed “the supernatural ability to stop time,” attack the matron ,and return to his worksite.

Lemon discovered this and other details as he pored over historical records, including a transcript of Williams’ short trial in 1931. And he could not help but wonder whether race played a factor in the jury’s decision to convict Williams in just a few hours.

“This was a stain on the court, a legal lynching in a sense, which is harsh thing to say, but that’s what it was,” Lemon said. “People are looking now not just at classic civil rights cases from the South, but also cases in the North where race played a factor in these unjust sentences.”

About seven years ago, he enlisted the help of attorney Robert Keller, who worked pro bono to help him try to get the case overturned.

Last summer, Keller and Lemon met with District Attorney Jack Stollsteimer, hoping that he might take an interest in the case. Stollsteimer agreed to take a look and read the trial transcript during his summer vacation in Cape May.

“It was just an alignment of the stars,” Lemon said. “The right time, right place, and the right change in politics in our county.” (Stollsteimer was the first Democrat to be elected district attorney in county history.)

Stollsteimer brought the case to President Judge Kevin F. Kelly, who agreed to hear a joint motion from Stollsteimer and Keller to have the case reopened.

In an interview Tuesday, Stollsteimer said several aspects of the prosecution troubled him. He cited “subversions of due process rights,” including what he called a “browbeaten confession” that the teen gave police after repeated interrogations without a lawyer or an adult present.

It stunned Stollsteimer, he said, that Williams was sent to the electric chair just a few months after his conviction, without any appeal filed by his attorney.

“In the criminal justice system, we’re in the business of making people’s lives better. That’s what we believe every day when we come to work,” Stollsteimer said. “But as long as humans are involved in this process, there will be errors, so if we can identify issues and right those wrongs, it’s in our interest to do that.”

During a hearing Monday, the lawyers laid out three decades of research and asked that the judge overturn Williams’ conviction.

They were joined in that effort by Robare’s great-grandniece, Theresa Smithers, who had done her own research and who testified from her home in Michigan that she believed Williams was not guilty.

“I know that Alexander did not commit the murder,” Smithers said, calling the teen a “handy scapegoat” for prosecutors at the time.

Kelly, on his final day as Delaware County’s president judge, concluded that evidence withheld by prosecutors could have changed the outcome of the trial. He seemed particularly troubled by the bloody handprint.

“In short, it appears with the imposition of his death sentence, Mr. Williams was legally abandoned by the court system and left to die at the hands of the state,” Kelly said.

He granted the request for a retrial, and Stollsteimer immediately withdrew the charges.

In so doing, Lemon said, the judge and prosecutor righted a wrong that had wounded many people across multiple generations.

“It feels astonishing,” he said. “It feels like a great weight has been lifted off my shoulders. I’m not resentful in any way, but that was a huge burden to carry for so many years.”