But one witness for the defense, Dr. Michael Moritz, a leading expert on salt poisoning at Children's Hospital of Pittsburgh, believes that, based on similar cases of children in foster care exhibiting behavioral issues and evidence of pica, he knows what happened to Andrew that day.
In his 2008 interview with ABC News, Moritz said, "I think [Andrew] was in one of his feeding binges. He was having a tantrum, and he was unsupervised for a brief period of time, and I believe that he ingested a large amount of salt."
In court, Moritz testified that he still believed it was a case of acute poisoning.
Furthermore, Moritz said he didn't believe Overton knew that Andrew was dying and that even if she had rushed to the hospital immediately, the child may not have survived.
"There is literature showing that even with today's medical technology, under the best medical care, even when it happens in the hospital, you die," Moritz told the court. "This is not necessarily preventable."
Prosecution experts, however, disagreed.
Heated Testimony at Hearing
Perhaps the most surprising testimony at the hearing came when the former prosecutor, Eastwood, admitted that by January 2011, she had concluded that she is an alcoholic and got sober. She also admitted taking diet pills while a prosecutor, but denied that her judgment was affected by use of alcohol or diet pills during Overton's trial. She also testified that she could not recall some specifics from the trial and she denied withholding evidence from defense attorneys at the time.
Eastwood's fragile and often soft-spoken presentation was in stark contrast to her demeanor when she spoke to "20/20" in 2008.
"I feel very confident that I did the right thing in presenting the evidence and having her convicted," Eastwood said.
At the time, she remained convinced that Hannah Overton knew or should have known that withholding medical treatment would kill Andrew.
"I think she was angry, enraged, with wanting to punish him and hurt him and then realized, 'Oh my gosh, I've really hurt him.'" she said.
Eastwood's co-counsel at the time of trial, Anna Jimenez, later became her supervisor and fired Eastwood for her alleged actions in an unrelated case. At the hearing for a new trial for Hannah Overton, Jimenez testified that she felt Eastwood was dishonest in disclosing evidence to the defense.
The defense has argued that evidence pertaining to Andrew's gastric content was deliberately kept from Overton's trial attorneys.
In her testimony, Jimenez said she feared that Eastwood had, in fact, withheld evidence. She said she raised her concerns about Eastwood's behavior to Eastwood's superiors but nothing was done.
"I was concerned there was possibility that Sandra may have been withholding this intentionally," she told the court.
The judge at the February hearing -- who was the same judge who presided over the original 2007 case -- ultimately concluded that Overton did not deserve a new trial based on the arguments presented at the hearing, but the final decision lies with the Texas Court of Criminal Appeals. Overton said she is confident the higher court will make the right call.
"I do believe that the Court of Criminal Appeals, when they look at the record, they will see that the record does not support [the judge's] decision," she said.
In a statement to "20/20," Eastwood's lawyer pointed out that every court that has considered Overton's claims of prosecutorial misconduct has found insufficient evidence to prove the allegations. Eastwood filed a motion for sanctions against Hannah Overton's attorneys for what she says are false allegations and false testimony, and it was denied by the trial court judge. Additionally, the state bar and the state attorney general declined to pursue any disciplinary actions against Eastwood after their own investigations. The exact nature of those investigations, and whether they have anything to do with the Overton case, are unclear, because they are not released publicly.
Overton's husband, Larry, has been a single father of five children while his wife has been incarcerated. Once a month, he packs them into the family van and travels 300 miles to the Texas prison where she is being held. There, Overton's contact with her children is limited. Because she was convicted of a capital offense, she's not allowed to touch them.
Overton said she tries to stay involved in her children's lives as much as possible, organizing their birthday parties and helping them plan how to redecorate their rooms.
Her youngest, Emma, was just an infant when Overton was jailed. Overton says family and friends put up photos of Overton in their homes so that Emma and her siblings are constantly surrounded by reminders of their mother. Today, Emma is 5.
Andrew would have celebrated his 10th birthday this past summer.
Overton told "20/20" in 2008 that she did not regret trying to adopt Andrew.
"I wouldn't take that away," she said. "He had brothers and sisters and a mommy and daddy, what he called his forever family, because we had to go through a lot of pain since then. It's not fair to him. Or to us."