Lost Child: What Happened to Etan Patz?

It's a mystery that began 30 years ago. On May 25, 1979, six-year-old Etan Patz disappeared in the Soho neighborhood of New York City walking to the school bus stop.

The case remains officially unsolved. For the Patz family, it has been three decades of waiting and agonizing investigations; years of wondering what happened to their blond son with the gorgeous smile and questioning why the man they say is responsible for his kidnapping has never been held criminally liable.

Watch "20/20" Friday at 10 p.m. ET for the full report.

For the world, Etan's disappearance marks profound changes in how we investigate missing children cases 30 years later and how we protect kids from pedophilia.

But it all began on what was otherwise a totally normal Friday morning, prior to the long Memorial Day holiday in 1979.

Stan and Julie Patz lived in a loft apartment in New York City, with their three children; Etan was the middle child. Their Soho neighborhood was one of Manhattan's less discovered areas -- a quiet community full of artists and adventurers.

"Things were quite different then," Stan Patz said. "There were a lot of other young couples with children -- it had the lowest crime rate in New York City ... mostly because there was no one here."

On the morning of May 25, 1979, Stan and Julie agreed to let six-year-old Etan walk the two blocks to his school bus stop alone. He had been pestering them for weeks for permission and the Patzs' reluctantly agreed.

"At some point in every parent's life they send their children to school alone. Did we do it too early? Obviously we did," Stan Patz said. "It was familiar territory. This was a very safe neighborhood. Every parent wants their children to be independent. They want them to be outgoing. When do you let them out of your sight? When they are 21?"

Julie Patz said goodbye to her son in front of their apartment building and then returned upstairs, tending to her younger son, who had a sleepover with friends at the house the previous night. The family was getting ready for a trip later that day.

Etan walked west on Prince Street, headed toward the school bus stop. But by late afternoon, when Etan had not returned home, the Patz's plans and lives drastically changed.

Missing Child: Search for Etan Patz Begins

Stan first learned something was wrong when Julie called him at a photo shoot uptown. Brushing it off as "nothing" at first, Stan got on the subway to come home. As soon as he walked into the loft and saw his wife, he knew something awful had happened.

"When I came through the door, I saw her... and she was pale," Stan Patz told ABC News. "And she said, 'I don't know where he is, I called the police.' Nothing has been the same since."

As police launched an investigation and a small army of volunteers searched for the missing child, Stan Patz, a professional photographer, turned to his trade; he went into his darkroom and made prints of the countless photos he had taken of Etan.

"I had all these pictures of Etan smiling, laughing, playing, fooling around. ... I went to a number of places where Etan might have gone that morning and showed the shopkeepers," he said. "I stopped people on the street. ... I just showed the pictures around."

The pictures would eventually go on "missing" posters that would be put up all over town and make their way around the country and even overseas.

Despite the community rallying behind the Patz family, the police had no leads. No suspects. At first, detectives investigated Stan and Julie Patz as people of interest in the disappearance of their son.

But, even after police cleared Etan's parents, it would be years before investigators had any substantive break in the case. The twists, turns and dead ends that followed have left the family aching for justice.

In the 30 years since Etan's disappearance, the pictures of Etan -- his personality leaping from the page of the "Missing" posters -- have kept this case alive and sparked a national campaign to find missing children.

Etan Patz's pictures have become a national symbol of lost innocence and a haunting reminder of the questions left to be answered.