A third person has been arrested in connection with a 4-year-old boy who died after allegedly being abused, Aurora police said Friday.

The boy, Alexander Jose Sarmiento, died May 16 after he was taken to the hospital with severe injuries. The boy was being supervised by Alexander Martinez-Armstrong, 24, who was watching the boy for his mother, Destiny Rose Lipsky, 23.

Martinez-Armstrong was arrested after he allegedly told police the boy’s severe injuries were a result of “discipline” he was giving the boy. Martinez-Armstrong faces a charge of murder, while Lipsky, who told police she similarly disciplined her son, is facing a charge of child abuse resulting in death.

The third person, who police arrested Thursday, was identified as Chyann Moon, 23, and is Martinez-Armstrong’s girlfriend, according to an arrest affidavit. The boy was in Moon’s apartment when the abuse occurred. She also faces a charge of child abuse resulting in death.

Martinez-Armstrong, who was not the boy’s father, told police he was watching him because he had been misbehaving.

Lipsky told police she entrusted her son to Martinez-Armstrong’s care so he could serve as a “boot camp” and correct the boy’s behavior, which she said included not listening to her, being disruptive and sneaking candy, according to the affidavit.

The alleged abuse that occurred over the four days Martinez-Armstrong watched the boy included hitting the boy with a belt and making him do pushups, planks and wall sits, according to the affidavit.

On the day of the boy’s death, Martinez-Armstrong told police he hit the boy 21 times with a belt, which included strikes all over his body. He told police he would hit the boy every time he made a “mistake.”

Moon told investigators the strikes continued up until an hour before they called 911 to report the boy was no longer breathing.