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.
"The next thing I know, a few hours later the dad and the owner came out and was crying," said Kenneth Brooks, an eyewitness. "When EMS had him, he was just limp, you know. Right there, I knew the kid was gone. It's a sad day."
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. Rosen's office says records show the boy had been listed as accounted for on a list of students compiled upon a return to the day care. An investigation is underway into the tragic oversight.
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 boy was later pronounced dead.
"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.
"I can't fathom being a father and losing a child like this, and so our prayers and thoughts are with this family," said Rosen.
According to records, the day care 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 day care 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.
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