An infant found dead on Monday by his baby sitter in a Johnson Ranch-area home day care died of untreated pneumonia, a preliminary autopsy report found.
Medical crews tried reviving Ethan Erwin, 2 months, before he was airlifted to a Mesa hospital, but they were unsuccessful.
A second infant also died on Tuesday in the care of a sitter at a home less than a mile away.
Kassidy Erwin, 11 months, was found not breathing while sleeping in a car seat inside the house.
She was also airlifted and was pronounced dead at the hospital.
The deaths are not linked. The infants share the same last name but are not related.
Kassidy's cause of death is still undetermined. Authorities are awaiting autopsy results from the Maricopa County Medical Examiner's Office.
The Pinal County Sheriff's Office is investigating both deaths.