Less than a week before his death, 5-year-old Garnett-Paul Spears’s sodium hit a lethal level without any medical explanation.

A couple of days later, hospital staff found him on his back. Unresponsive. Barely breathing. Pupils blown. And his skin was light gray in color.

An EEG indicated brain death. He was declared dead on Jan. 23.

Garnett’s medical records speak to years of sickness in his short life — severe ear infections, high fevers, seizures, digestive problems.

Now, authorities believe there may be an explanation.

They believe it may have been his mother who was ill, possibly suffering from Munchausen syndrome by proxy, a psychiatric illness in which a parent makes her child sick to get attention or sympathy.

Prosecutors told local media they believe 26-year-old Lacey Spears fed her son dangerous amounts of salt after she conducted research on the Internet about its effects. She has been charged in Westchester County, N.Y., with second-degree depraved murder and first-degree manslaughter, according to her indictment. On Tuesday, she turned herself in to police.

She pleaded not guilty the same day.

“She really didn’t show any emotion,” Westchester Police Capt. Christopher Calabrese told CBS2. “She was kind of stoic when she came here. I think that she knew the grand jury was going on. She anticipated this happening, and she turned herself in with her attorney.”

Investigators suspect the single mother may have poisoned her child at least twice, once before his seizures and again when his sodium spiked. Both incidents occurred in January. But they also suspect his medical abuse went back much further — fueled by the social media attention she gained on Facebook, Twitter and blogs, according to local news reports.

When Garnett was 5 days old, he reportedly went to an Alabama hospital with a high fever and ear infection. He had fluid in his ears, according to Spears’s statement to police.

At 10 weeks, she said he stopped breathing at the same time his sodium level climbed. At the time, doctors could not explain why.

Garnett purportedly had trouble gaining weight and, at 9 months, doctors put in a feeding tube.

By age 2, he was in and out of emergency rooms in Florida — once with a staph infection and blood leaking from his ears and his nose.

In her statement, Spears spewed medical and hospital jargon while describing her son’s medical woes, according to the indictment.

In January, when Garnett’s sodium hit a suspiciously high level, doctors at Maria Fareri Children’s Hospital in Valhalla, N.Y., alerted police. Witnesses told the Journal News they saw a doctor confront Spears, saying it was “metabolically impossible” for her son’s body to produce that much sodium and that “something isn’t right.”

Days before Garnett died, he was reportedly awake and talking when friends came to visit. One friend said the boy told her, “Don’t leave me.”

The indictment stated Spears stayed in Garnett’s hospital room.

Assistant District Attorney Doreen Lloyd said Spears took her son into the hospital room’s private bathroom and injected the sodium into the feeding tube in his stomach.

“This mother was intentionally feeding her son salt in toxic levels,” he said.

A neighbor said Spears called her from the hospital and told her to get rid of a bag she used to feed him through his tube. Police later recovered that bag, which had extremely high levels of sodium, the Journal News stated.

Spears told police she sometimes used “a pinch of salt” for flavor when feeding her son and that once in a while he would play with the syringe she used to feed him, claiming it was possible he put something in his tube.

Rambo, N.Y., Detective Kirk Budnick said it was possible the cause of death may never be known. At that, Spears allegedly smiled and appeared somewhat relieved, according to the indictment.

Spears said that if she gave her son the wrong medication or if she mixed in something that killed him, it was not her fault because those medications were prescribed. She said she did not murder him.

If convicted, Spears faces a maximum sentence of 20 years to life in prison.