Casey Anthony 'Will Never' Identify Caylee's Father, Says Source: 'She'll Take That Secret to the Grave’

Casey Anthony was an unmarried 19-year-old in 2006 when she gave birth to her daughter, Caylee. She wasn’t seeing anyone seriously and had dated several guys in the months leading up to her pregnancy.

So when she faced trial in 2011 in Caylee’s 2008 death, the question of paternity arose in the courtroom.

During the trial, defense attorney Jose Baez elicited gasps when he implied that Anthony’s brother, Lee, could have been the girl’s father. The suspicion compelled the FBI to test Lee Anthony, but it was revealed he wasn’t the father.

Ten years after Caylee was last seen alive, the question remains: Who was her father?

Even those close to Anthony have no idea.

“I don’t know who the father was,” a source close to the now-32-year-old Florida woman tells PEOPLE. “She doesn’t talk about it. She will never tell anyone. She’ll take that secret to the grave.”

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In a 2008 police interview, Anthony’s neighbor Brittany Schieber said that Anthony was seven months along before she realized she was pregnant. According to Schieber, Anthony said that she didn’t know who the father was, and “that it was just a random one night thing.”

Another friend, Lauren Gibbs, told detectives, “Um, well, at first I thought it was Jesse. I don’t know his last name. But um, and then she told me later that it was not and that it was somebody who was in the Army or something like that. But then she told me that he had died last year in a car accident.”

Jesse Grund, Anthony’s on-and-off boyfriend at the time of Caylee’s birth, told PEOPLE in 2009 that he thought it was “possible” Caylee was his, but a subsequent DNA test proved he wasn’t the father.

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Even Anthony’s parents, George and Cindy Anthony were unsure of Caylee’s paternity — and did not realize that their daughter was pregnant until the third trimester.

When the family showed up to the 2005 wedding of Cindy’s brother, Anthony was seven months pregnant — with her bump protruding in a tight dress.

Cindy’s brother, Rick Plesea, confronted his sister about Anthony’s pregnancy. “She was in complete denial,” he later told police of Cindy. “Cindy said, ‘Casey told us that she’d have to have sex in order to have a baby and that she did not have sex with anyone.'”

After her 2011 acquittal on murder charges in Caylee’s death, Anthony has kept a low-profile and has only once spoken publicly about the case. In the Associated Press interview, she gave no indication of who the father of her child is.

According to the source close to Anthony, she doesn’t speak much about Caylee, who would be turning 13 this August. “If she knows who the father is, she’s not telling,” says the source. “She definitely won’t ever say anything publicly. I doubt anyone will ever know.”