In her only interview, Spears explains to “48 Hours,” that Garnett’s health issues began just weeks later and he was diagnosed with failure to thrive. “He was in and out of the hospital for the first nine weeks of his life, and then -- even for a period of time afterwards.”

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Plagued by health issues, Lacey Spears took Garnett to over a dozen medical facilities. In her only interview, she told “48 Hours,” “Garnett had severe ear infections ... and the biggest problem we had was we couldn’t get him to eat ... so he was losing weight...he just would not take a bottle, he would not take baby food, he would not nurse.”

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At 10 weeks old, doctors at a hospital in Alabama found his sodium levels to be unusually high at 180. The normal range is 140 for infants.

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According to Lacey Spears, she still couldn’t get Garnett to eat and gain weight. So she took him to more doctors and at 9 months old, he had surgery to insert a feeding tube commonly referred to as a G-tube, where nutrients go directly into an opening in the abdomen, either through a hanging bag or a bottle or through a syringe.

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Lacey Spears documented her life with Garnett on social media. She included photos of him during hospital stays and frequently posted on sites like Facebook, Twitter, and a blog she created, “Garnett’s Journey.”

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“48 Hours” obtained documents that revealed suspicions that Lacey Spears suffered from Munchausen syndrome by proxy, a condition where illnesses may be faked or induced in someone else, usually a mother onto her child, and it’s mostly to seek attention.

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In 2012, Lacey and Garnett Spears moved to Chestnut Ridge, N.Y. They lived in an alternative and holistic living community called The Fellowship Community, where Spears worked taking care of the elderly and Garnett attended school.

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Life at the Fellowship was going well for Lacey Spears, and for Garnett, who after living there for a year-and-a-half, was thriving. “He was a little social butterfly and he loved it,” Spears tells “48 Hours.”

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Shortly after Garnett Spears turned 5, he was hospitalized once again; this time never returned home. Lacey Spears tells “48 Hours” that Garnett was experiencing seizures.

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Lacey Spears sits on a hospital bed with her son Garnett on Jan. 17, 2014, after he was admitted into New York’s Nyack Hospital. A video EEG was set up using a special cap on his head, wired to a machine that monitors brain activity. Meanwhile, with his mother’s consent, a camera recorded Garnett in the room.

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Everything seemed normal but almost two days after being admitted, he took a turn for the worse. Tests later showed that Garnett’s sodium levels, which were a normal 138 when he was admitted, rose to dangerous levels of 182. This is a frame of the video EEG footage before Garnett’s condition changed.

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WATCH: What happened in the hospital bathroom?

Garnett Spears was airlifted to Westchester’s Maria Fareri Children’s Hospital. His condition improved but the recovery lasted less than 24 hours. The high sodium levels had already caused Garnett’s brain to swell. He was placed on life support. In her only interview, Spears she tells “48 Hours” she couldn’t understand why. “I don’t know what’s happening ... this is my 5-year-old son on life support.”

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On Jan. 23, 2014, Garnett Spears was officially declared dead. Doctors suspected Lacey Spears poisoned her son with salt and called for a police investigation. Police immediately acquired search warrants to enter her home and they seized hundreds of items including the bottle of sea salt seen in this photo.

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During the second search warrant, almost two days later, police returned to The Fellowship Community. Two feeding bags were recovered. Lab reports later reveal they contained massive amounts of sodium. Each bag had at least, according to officials, the equivalent of 69 small packets of salt. Retired Westchester County Det. Daniel Carfi said, “The amounts of sodium that were in those bags were lethal.”

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Lacey Spears was arrested on June 17, 2014, almost five months after her son’s death.

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WATCH: In first interview, Lacey Spears talks about the loss of her sonhttps://www.cbsnews.com/news/lacey-spears-a-mother-accused/

Lacey Spears’ trial began on Feb. 3, 2015. Her legal team insists that Garnett’s death was caused by a medical mistake with Nyack Hospital at fault. Stephen Riebling, right, one of Spears’ attorneys, tells “48 Hours,” “They [Nyack Hospital] changed his diet that caused or exacerbated his stomach problem ... they didn’t give him the necessary medication during the day to combat dehydration. Then they exacerbated the dehydration by giving him a rapid infusion of IV solution.”

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On March 2, 2015, after less than three days of deliberations, jurors found Lacey Spears guilty of second-degree murder.

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Lacey Spears was sentenced to 20 years to life in prison. She will be 46 when eligible for parole.

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WATCH: Death of 5-year-old case police they won’t forgethttps://www.cbsnews.com/news/lacey-spears-a-mother-accused/