It's one of the biggest mysteries hanging over the Casey Anthony murder trial: Who is Caylee's biological father?
Two of Casey's ex-boyfriends have testified at the trial, but neither of them is Caylee's dad.
So could the father be Casey's ex-fiancé, Jesse Grund? He says Casey told him he was little Caylee's dad while she was pregnant but he revealed to INSIDE EDITION's Paul Boyd that a paternity test proved otherwise.
"[A paternity test] said that there was zero-percent probability that I was her biological father," Grund said.
Casey suggested to the FBI that her own brother Lee may have been Caylee's dad.
There was even speculation the brother and sister's first names, Casey and Lee, were combined for the little girl's name, Caylee
But a DNA test also proved that Lee was not Caylee's father.
At another point during the investigation, Casey claimed the father was a "really hot guy" with whom she had a one-night stand.
The mystery man's name was said to be Josh. He came from Georgia and was married.
"Casey ultimately says this person died in a car crash, yet she has no obituary to show even the name," says True Crime's Aphrodite Jones.
The Anthony family has stood by her story. In a 2008 interview, Lee said, "He's passed away. I know that she met this person through some mutual friends […] We will never release the name of that person that has passed away or his family, out of respect for that family."
But Aphrodite Jones has another theory:
"Based on Casey's lying history I would say all of this is trumped up and she doesn't know who in fact who the father was."