MISSING: Michelle Otter, 15, disappeared after leaving for school

OCALA, Fla. - It’s a mystery that has stumped Marion County deputies for 24 years: what happened to Michelle Otter? 

"That day she left the house and was never seen again," said Marion County Sheriff’s Office Sgt. Paul Bloom. 

It was May 7, 1998, in Fort McCoy, a rural community north of Ocala. 

Michelle left her family’s home to go to school - possibly to catch the school bus. What happened to her next is a mystery. 

"Initially it came in as a runaway possibly…15-year-old female that had left home. Had not come back home from school," Bloom explained. 

Sgt. Bloom says that theory was quickly thrown out. Investigators say Michelle never showed up at North Marion High School at all. 

And as the days passed by, her disappearance grew more and more sinister. 

"She took nothing with her… there was money on the dresser, all of her clothes at the house. Now we are suspecting something more heinous," Bloom said.

Days have turned into years without any answers. 

"One of the biggest challenges, especially when working on a case such as a long-term missing child case is keeping that case active in the public eye," said John Bischoff, VP of the Missing Children Division at the National Center for Missing and Exploited Children (NCMEC). 

Bischoff says most of the cases reported to them are resolved quickly. But we found some Florida kids’ disappearances date back to the 1960s. 

The organization has a team devoted to showing the public how these kids may look today. 

"They take pictures of the missing child, they take pictures of the mother, the father around that age, and they work to make a composite – an age progression," Bischoff said. 

Michelle would be 39 years old now. 

Little is known about her and what she was like as her family declined an interview with FOX 35. Deputies say shortly after the teen went missing, the family's trailer in Fort McCoy burned down. Investigators didn’t find anything indicating the fire was suspicious. 

"There is a missing piece to this puzzle. The puzzle to solving her disappearance. Somebody has that missing piece, they’re holding onto that," Bloom said. 

Anyone with information on Michelle’s disappearance is asked to call the Marion County Sheriff’s Office or the NCMEC tip line at 1-800-THE-LOST. 

Get the most up-to-date forecast, sign up for weather alerts, and view live interactive radar for Orlando and Central Florida.

Watch FOX 35 News for the latest launch updates.