"I do believe siblings have gotten together, substantiated a story that they can't even keep straight, that the lies lead to lies, that the admissions keep modifying," Sirgo said.
Overstreet was interviewed by several Florida investigators earlier this week at the FBI headquarters in Nashville, Tenn., but has always denied any involvement.
Sheriff Hardy confirmed that Overstreet has been interviewed regarding Haleigh's disappearance but said that he has "lawyered up."
Haleigh was first reported missing by her father, who made a chilling 911 call when he discovered her missing.
"I just got home from work and my 5-year-old daughter is gone," he told a dispatcher. "If I find whoever has my daughter before you all do, I'm killing them. I don't care if I spend the rest of my life in prison."
Cummings had left Haleigh and her 3-year-old brother in the care of Croslin, who was 17 at the time.
"She was sleeping right next to me," Croslin told the Palatka Daily News. "I can't believe I didn't hear anything."
Cummings told police that when he returned from work early that Tuesday morning, the back door was propped open and Haleigh was gone.
"Somebody came in my back door, broke into my home and stole my daughter," he said.
"I don't know why somebody would take her," Crystal Sheffield, the girl's mother, said at the time of her daughter's disappearance. "I'm scared for her. She is probably scared and cold and hungry."
Throughout the ordeal, members of Haleigh's family have shown dogged determination.
"If I lost hope, what does she have left then?" Ronald Cummings told ABC News' Orlando affiliate WFTV in February 2009. "Like I gave up on her? I'm not giving up on her. Never."
Florida police treated Haleigh's disappearance as an abduction, investigators said early on in the investigation, after concluding she did not simply wander off alone.
"All the world is a suspect," Det. John Merchant of the Putnam County Sheriff's Office told reporters at the time of her disappearance. "We are going to treat everybody, every family member, every associate, like a suspect until we eliminate them."
As police launched a massive search effort that included divers and K-9 units, Sheffield begged for her daughter's return.
"Whoever has her, I know you're watching," she told "Good Morning America" in February 2009. "She ain't done nothing wrong. Please bring her back."
In June 2009, vowing to leave no stone unturned, investigators dug up much of the 30-acre Glen St. Mary home of Haleigh's grandmother, Marie Griffis, but only recovered animal remains.