CINCINNATI -- It’s hard to imagine the agony a family goes through when a child disappears.
David Markham has been living that nightmare for the past six years.
His daughter, Katelyn, disappeared from Fairfield in 2011. She was 21 years old.
Initially, Markham said his thoughts went from being in denial to trying to figure out a logical explanation as to why his daughter vanished.
But there was no explanation.
“You always want to have that little ounce of hope that she's going to walk in that door some day, but deep down in your heart you know that something's wrong,” Markham said.
Hundreds of people joined organized searches for the woman in the weeks and months after she disappeared. Thousands of Tri-Staters prayed she would come home safe.
But all hopes ended 20 months after Katelyn vanished, when the skeletal remains of the vivacious art student were found in a wooded area in rural Cedar Grove, Indiana, 30 miles away.
From The Vault: Watch Katelyn Markham's tragic story
“You're helpless that you can't take care of your child,” Markham said.
The family of Paige Johnson still doesn’t know where she is. She disappeared from Covington when she was 17.
Friends and family gathered at Devou Park last weekend on the anniversary of her disappearance.
Alicen Franks, Johnson’s cousin, stood with a heavy heart at a memorial tree.
“It's pretty terrible,” Franks said. “It's kind of sometimes a walking nightmare -- waiting -- not knowing for sure where she's at.”
There are about 80,000 reports of missing children every year, according to the National Center for Missing and Exploited Children. That’s 2,000 reports a day.
Protocol combined with widespread reach through social media has helped; Amber Alerts have brought over 800 children back home.
But it’s the dogged determination of detectives like Cincinnati Police Department’s Jason Hodge who work to reunite children with their families.
Hodge works within the Cincinnati Police Personal Crimes Unit, which has 81 missing cases.
“The most difficult would be cases where we simply just run out of leads,” Hodge said. “They've disappeared off the face of the earth.”
The case of Esme Kenney forever changed how the department handles missing children cases.
Convicted serial killer Anthony Kirkland murdered the 13-year-old in 2009.
Kirkland kidnapped Kenney as she jogged alone around the Winton Hills reservoir close to her home. Police were out looking for her when they came upon Kirkland in the woods. He had Kenney's iPod and her watch. They found her body nearby.
Police developed the Esme Procedure from Kenney’s case.
“It's very helpful because it gives me the opportunity to look into, for example, social media sites, where a lot of these missing people do put a lot of information out and I can use that as a tool to try and locate them,” Hodge said.
Butler County Sheriff Richard Jones said cases that involve young adults wandering away or getting mad at parents and leaving usually have happy endings. But this is not always the case.
“Then, we'll have the ones that are teenagers and they'll just disappear,” Jones said. “They hitchhike, they leave, the hook up with somebody on social media … and those are very difficult.”
Jones knows the case of Katelyn Markham all too well. Her photo is still plastered on posters of unsolved Butler County murders.
Markham hopes whoever is responsible for his daughter’s death listens to their conscience.
“It's very selfish of that person to live their life and wake up every morning with total disregard to Katelyn, or her sister, or me, or her friends or anybody else,” he said.
Web Editors Abby Anstead and Greg Noble contributed to this report.