The boy has been identified as Ricky Thao.
The boy's mother told officials the boy had been playing outside with his sisters, and his sisters reportedly heard him jump into the family's inflatable pool, containing about three feet of water.
The boy's mother told officials the boy had been in the pool many times and knew how to swim.
It is unclear how long the boy was in the pool before he was discovered unresponsive.
Neighbor Christina Pena talked with FOX6 News Monday and recalled the incident.
"The first thing I seen was little feet. Seen the father pumping on the boy's chest and I'm like, 'oh my god, that's the little boy.' I heard the mother screaming, and I feel sorry for the mother and the hell she's going to have to go through," Pena said.
Pena told FOX6 News about 13 years ago, her own 16-month-old son drowned in a bathtub. She described the emotional torture and grief experienced at the hospital before her son died.
"They're telling you one by one each organ that's shutting down, and then the heart stops. You have the choice to revive them, or you don't, and then it gets tough at the end, like mine did. After 12 hours I couldn't watch that anymore," Pena said.