A 5-year-old child has died after being left in a hot car for several hours while his family prepared for a birthday celebration, according to Texas authorities.

Sheriff Ed Gonzalez of Harris County, Texas, released a statement on his Facebook page, confirming that the child was in the car for approximately two to three hours and strapped to his car seat. He said the mother of the child assumed that the two of her children, an 8-year-old and a 5-year-old, had both exited the car after returning home Monday.

“Upon arrival units determined a child (possibly 5 yrs) was left inside a vehicle for several hours. The child was pronounced deceased at the scene. HCSO Investigators are en route to the scene,” Sheriff Gonzalez shared. “Preliminary info: mom returned home with kiddos in tow; 8 yr old and 5 yr old. 5-yr-old was strapped in his child safety seat in the back. They exited the car, saw the 8-yr-old and assumed the other child was out also.”

Sheriff Gonzalez concluded that the child usually knows how to unbuckle himself from his car seat, but he did not know how to as this was a loaner car. The child’s mother discovered he was still in the car hours later.

“Child (5-yr-old) typically knows how to unbuckle himself and exit on his own. But this car is said to be a loaner car,” Sheriff Gonzalez said. “Family was making preparations to celebrate the 8-yr-olds birthday party at the house later today. After 2-3 hours, mom realized the child (5-yr-old) was nowhere to be found. She ran outside and discovered her son still in the safety seat. EMS was called and the child was pronounced deceased at the scene. Investigation is ongoing.”

According to ABC News, a scorching 102 degrees was recorded in Houston on Monday, marking the hottest temperature this early in the summer since 2011. The sheriff’s office said investigators would meet with the Harris County District Attorney’s Office to present their investigation findings.

KidsAndCars.org reports the boy is the fifth American child to die in a hot car this year.