Left inset: Artnesia Aaliyah Baptist. Right inset: Alexsander Byrne (Maricopa County Sheriff's Office). Background: The Arizona residence where Artnesia Aaliyah Baptist and Alexsander Byrne were staying when they killed Baptist's 22-month-old son (KPNX/YouTube).

An Arizona mother and her boyfriend are facing murder and child abuse charges after allegedly pouring scalding hot water over the head of the mom's toddler and keeping his injuries secret until the boy died from a severe infection and sepsis.

Artnesia Aaliyah Baptist, 24, and Alexsander Byrne, 21, are charged in Maricopa County with killing Baptist's 22-month-old son in April. The Mesa couple then "hid" the boy's burns from his biological father and threatened to kill the man if he tried to pick the child up.

Byrne later admitted that they "intentionally refused medical treatment for the child" because they thought Child Protective Services would get involved.

"[Byrne] admitted that he knew it was wrong, illegal, caused additional injury to the child, additional pain, and caused the death of the child," a probable cause affidavit obtained by Law&Crime says.

The document accuses Baptist and Byrne of causing "sustained severe liquid burns" on April 3 that appeared to be "similar to hot/boiling water poured over the victim's head, which ran down the victim's neck, shoulders, chest, back and hips and caused the skin to loosen and blister."

Police say 40% of his body was burned.

Byrne allegedly reported seeing "big chunks" of skin peeling off the child during a diaper change. Baptist and Byrne both denied knowing what happened to cause his burns.

An inspection of Byrne's internet history showed searches such as "can 2nd degree burns kill you" and "first degree burns," according to the affidavit. Baptist and Byrne allegedly told Byrne's parents the boy had a cold, and when his mother offered to take the child to an urgent care, they lied and said he was "getting better."

The couple tried to treat the injuries with burn cream, Tylenol and aloe vera, according to the affidavit. Byrne allegedly told police that Baptist's son had a fever, started throwing up, and was unable to walk after being burned.

Police say the child died from a severe infection and sepsis on April 12.

"The infection continued to spread and grow causing what the medical examiner described as a slow, painful decline of sepsis that had occurred at least 48 hours prior to the child's death," the affidavit says.

Authorities were notified about his death and responded to a home where the couple was staying around 9 a.m. that Sunday. The child was pronounced dead at a local hospital.

"The smell of his skin filled the emergency room with the odor of rotten flesh," the affidavit says. "The skin on the victim's shoulders, clavicle, and down his sternum, under his hair, down the back of his neck, his back, buttocks, and hips, was missing/red and appeared infected."

Baptist and Byrne were both arrested this week following an investigation. They are each facing charges of murder and child abuse, and are being held on a $1 million cash-only bond.