A Parent's Nightmare: Sex Offenders in Our Midst

April 13, 2010 — -- There are monsters in our midst. And they live in plain sight.

It is a growing terror for parents in the 23 states that allow registered sex offenders to reside within 1,000 feet of a school.

"Our children should be allowed to walk to school without worrying that a monster is going to jump out and steal them and never let them come home," said Diena Thompson, who lived a mother's worst nightmare.

Click HERE for resources for parents to help protect kids from sex offenders.

Seven-year-old Somer Thompson delighted in dancing three beats behind the rhythm of any song. Growing up in the warm embrace of her Florida neighborhood, Somer spent her days playing outdoors and dreaming of becoming a ballerina.

Somer's family affectionately nicknamed her "Grace."

"Because she was so klutzy," said Somer's mother, Diena Thompson. "I mean, she could not walk and chew bubble gum at the same time, but she loved to dance."

But there was nothing awkward about Somer when it came to showing her love for family and friends.

"I mean, she just wanted to hug everybody. She just always wanted everybody to be happy with her," said Thompson.

As a radiant but sensitive child, it wasn't surprising that Somer took it to heart whenever she got teased.

"She had problems with her speech. And she was in some special classes to try to help her and they made fun of that fact," said Thompson. "Children can be really, really mean."

On Oct. 19, 2009, that mockery became a launching point to tragedy. Somer was walking home from school with her siblings and some friends when another student made fun of her. The two exchanged words. Upset, Somer ran ahead of the group and disappeared among the sidewalk crowds.

When her sister and twin brother arrived home just 15 minutes later, Somer was nowhere to be found.

Within hours, hundreds of residents of Orange Park, Fla., were in the streets searching, many still hopeful that it was all somehow a mistake.

But by evening, Diena Thompson was experiencing every parent's worst agony. Somer had vanished. And the mystery on everyone's mind: How could this have happened without anyone seeing a thing? For Somer's neighborhood seemingly had the wholesomeness of a 1950s television series. A place where adults kept a watchful eye out for trouble, and strangers were easily spotted.

Somer Thompson: Where Sex Offenders Live

Just 48 hours after Somer went missing, the world finally heard the news Diena Thompson already knew in her gut.

"Around 3 p.m. Wednesday, investigators discovered the body of a small girl among the garbage," reported local news station WJXX-TV.

Within hours of finding Somer's body, police focused on an empty house that was under renovation and began processing it as a crime scene. The witness who last saw Somer reported spotting her near the home.

It turned out one of the men who worked at the vacant house was a registered sex offender. He was later cleared of any connection to the Somer Thompson case.

However, the investigation did reveal something very important. After Somer Thompson disappeared, the residents of Orange Park were stunned to learn that their children had been walking through a virtual minefield of registered sex offenders each day: 162 living in just a five-mile radius of Somer's home.

Diena Thompson said people were quick to judge her because she was a single, working mother who let her kids walk by themselves to school.

"That's one of the things that hurts me the most," she said. "People want to... speculate, and think bad things about me."

Yet Florida state policy dictated that Thompson's children couldn't ride the school bus, because the family lived less than a mile from school.

"That makes no sense to me," said Thompson. "You're making my child walk in a jungle of monsters every single day, walking home from school."

Within days, police had accounted for the whereabouts of all registered sex offenders in that area. But Diena Thompson couldn't shake the feeling that the killer was still nearby, and watching.

"Every person I see, I'm in my mind, 'Hmm, I wonder,'" Thompson said.

Watch the full story on "20/20" Friday at 10 p.m. ET