A 3-year-old boy was found dead in a bus outside his day care in northwest Houston after he was apparently left inside following a field trip as temperatures outside soared to nearly 100 degrees.

The boy was one of 28 students from the “Discovering Me Academy” who were taken by bus Thursday to a nearby park for a field trip earlier in the day, the Harris County sheriff’s office said.

Houston police said during a Friday press conference that the children were loaded into the van around 10 a.m. and went to Doss Park. The group returned to the day care between 2 and 2:30 p.m.

School records show the 3-year-old had been listed as accounted for on a list of students, the sheriff’s office said. It was only when the boy’s father came to pick him up at 6:30 p.m. that officials discovered the boy was still on the bus.

He was found unresponsive, and the temperature inside the bus was 113 degrees, the sheriff’s office said.

Investigators believe the child was left inside the bus for at least three and a half hours. The boy was transported to the hospital and pronounced dead at 7:45 p.m.

Police are looking into the actions of the two day-care workers and the facility as part of their investigation, police said during Friday’s press conference. CNN has reached out to the academy for comment.

Deputies are interviewing the driver of the bus and a chaperone who was on the field trip, and both are said to be cooperating with the investigation.

“It’s just tragic,” said Constable Alan Rosen.

He also urged parents and caregivers to always look in and around their vehicle before leaving it.

“Do something that always reminds you that you have a loving package in the backseat of that car,” Rosen said.

In the US, hot car deaths are a consistent problem. An average of 37 children die each year from being left in hot cars, according to the safety organization Kids and Cars.

According to NoHeatStroke.org, Texas had the most such deaths from 1998 to 2015, and the month of July is usually the deadliest month.