ST. LOUIS (AP) — St. Louis County police are investigating the death of a 1-year-old boy who may have been left inside a van for several hours.

Officers were called around 6 p.m. Monday to the Casa Dia Montessori school in south St. Louis County to a report of an unresponsive child who was not breathing. One-year-old Tate Mitchell of St. Louis was pronounced dead at a hospital, St. Louis County police spokesman Shawn McGuire said. An autopsy was completed Tuesday, but details were not released.

The school said the child attended there. In a statement, school president Rick Deeba said the mother arrived at the school around 4:55 p.m. with the child already unresponsive in her van and brought him into the school. The school called 911 and CPR was administered.

McGuire said police received a call that a child had been left in a vehicle since the morning and was not breathing. He did not elaborate on the circumstances.

A police investigation is ongoing.

The high temperature in the St. Louis area on Monday was around 80 degrees, but the temperatures inside a vehicle with the windows up could have topped 100 degrees within a half-hour, the National Weather Service said.

Thirty-nine children in the U.S. have died of vehicular heatstroke this year, according to the advocacy group More than 800 children have died of heatstroke in vehicles since 1990, the organization said.

AP reporter Heather Hollingsworth in Kansas City, Missouri, contributed to this report.