A father arrived at his toddler son's daycare to pick him up on Thursday to discover him dead after he'd been left for four hours in the center's sweltering van.

The three-year-old boy, who has not been named, was left on the bus at Discovering Me Academy in Houston, Texas, on Thursday after returning from a field trip between 2.30pm and 3pm.

In the four hours before he was found, temperatures soared to 113F (45C) inside the vehicle, the Daily Mail reported.

Staff were looking for the boy when his father arrived, they found him while his father was there. Photo / KTRK
Staff were looking for the boy when his father arrived, they found him while his father was there. Photo / KTRK

When his father arrived to pick him up at 7pm, staff were still looking for him.

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They discovered him still inside the van when his father was there.

Afterwards, witnesses said they heard the man screaming in the parking lot.

It remains unclear why the boy did not get off the bus as he should have.

There were 28 children on the bus at the time and staff claim that the toddler was accounted for when he returned from the trip.

Overnight, the day care's bus driver, director and the chaperone who was also on the van were questioned by police.

No charges have yet been filed. Harris County Constable Alan Rosen said it was an example of "gross negligence".

He told ABC the case was his 'worst nightmare' and said it was an "absolute tragedy".

In 2013, the daycare center was cited for not having technology in its vehicles which forces drivers to walk through the entire bus before getting off to ensure that no child is left behind.

The technology triggers an alarm once the ignition has been switched off which can only be turned off at the back of the vehicle.

The idea is that the driver must walk the entire length of the bus to silence it and encounter any child that has been left behind accidentally on the way.