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The Precinct 1 Constable's Office says the child was found unresponsive in the van around 7 p.m., when his father arrived at Discovering Me Academy in the 8000 block of Antoine to pick up his son.
Two people -- the driver and a chaperone -- were detained for questioning.
Constable Alan Rosen says the child was among a group of 28 children who participated in a field trip on Thursday.
Deputy constables said it appeared the child was left in the vehicle for at least four hours. Temperatures inside reached at least 113 degrees, Rosen said.
When EMS workers arrived, they began to work on the child. He was transported to the hospital, where the tot was later pronounced dead.
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"It seems to me this was just gross negligence," Rosen said. "It's just tragic."
Rosen says the death was completely avoidable, and vowed to get "all the answers" for the family about how the tragedy occurred.
The victim has not been named.
According to records, the daycare was cited for several violations involving their van in 2015. One violation included not having an electronic child safety alarm, which is used to notify a driver that a child was left in the vehicle.
The daycare was also cited for not reporting a wreck involving the van in a timely manner, and for a driver not knowing the number of children in her group.