Etan Patz would be 52 now, far older than his parents on the day he disappeared.

And yet his story remains unfinished, unclear, unquiet. The smiling face of the 6-year-old boy from countless “Missing” posters in 1979 — a year that rewrote the norms of modern parenting — returns, yet again, to announce a new twist in the case that seems to never end.

That twist arrived Monday when a federal appeals court overturned the 2017 conviction of Pedro Hernandez, a troubled former stock clerk at the bodega near Etan’s home in SoHo where he disappeared on the way to school.

The court found fault with the trial court’s instructions to the jury in 2017 and ordered either a new trial or Mr. Hernandez’s release, a decision that will fall to the Manhattan district attorney, Alvin L. Bragg.

The arrest of Mr. Hernandez in 2012 followed untold hours of police investigation that spanned decades. But then there was a mistrial — hung jury — and, only after a second trial, a conviction. The jury foreman said the deliberations had been fraught. Now, eight years later, there is the prospect of a third trial in a case that can’t seem to stay closed.

“Jesus Christ,” said Louis K. Meisel, reacting to the decision and probably speaking for many. He owns the art gallery that Etan walked past for the last time on that fateful morning. He has been involved in the investigation since it began. He saw nothing and knew nothing, but the whole terrible thing happened on his turf.

“His mother watched him cross the street, and I owned the rest of the street,” he said. “I don’t know what to say. I’m surprised as hell.”