On Tuesday, a Virginia appeals court fully absolved Marvin Grimm Jr. of the 1975 crime, saying that new evidence had dismantled the state’s original case against him.

On a Saturday afternoon in 1975, a 3-year-old boy wandered into the woods behind the apartment complex where he lived in Richmond, Va.

Four days later, the toddler’s body was found floating in shallow water in the James River, fully clothed, arms folded across his chest, nine miles from home.

From there, the horror only grew: Forensic experts told the police that the boy had a gob of semen in the back of his throat and determined that he had been strangled during a sexual assault. Alcohol and a muscle relaxant were found in his blood, according to court documents.

About a month later, in December, the police arrested Marvin Leon Grimm Jr., a 20-year-old Navy veteran, picking him up after his shift at a carpeting company and questioning him for nine hours. Mr. Grimm, who had no criminal record, lived across the hall from the boy’s family and had argued with them over minor issues like toys that were damaged by his lawn mower. He was married, and his first child, a son, had been born 10 days before his arrest.

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Much of the key evidence that linked Mr. Grimm to the killing was the work of Mary Jane Burton, a senior analyst at Virginia’s crime lab who died in 1999.

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But in a recent podcast investigation of Ms. Burton’s work, a whistle-blower revealed that she had tried to alert officials that Ms. Burton was cutting corners and falsifying results, and an independent expert said that the serology evidence that Ms. Burton had analyzed in one of her own cases excluded the man who went on to be convicted of the crime. The podcast also noted that there were several accounts of shoddy work by Ms. Burton, and a perception that she favored the prosecution.

The podcast, “Admissible: Shreds of Evidence,” has triggered state reviews of about 4,800 cases handled by Ms. Burton. But the Grimm case was still jarring, said Tessa Kramer, the journalist who produced it.