Officials say one of the twins hospitalized after being found in a hot tub over the weekend has died.
Firefighters responded before noon Sunday after the children, a boy and a girl, were found in the backyard of a home near Warner and Priest.
Thursday, Tempe police said the boy had died. His sister continued to improve at the hospital, officers said.
The parents were home at the time the two went into the tub, firefighters said.
Police are investigating the incident but said Thursday what happened was a "terrible tragedy."