HOUSTON (AP) — A 5-year-old boy has died after being left in a vehicle in the Houston area as his mother prepared for a birthday party and as temperatures soared past 100 degrees, authorities said.

Harris County Sheriff Ed Gonzalez said the boy died Monday as his family was getting ready to celebrate his 8-year-old sister’s birthday. The boy, his sister and his mother went to the store to buy some items for the party, and when they returned home, the mother assumed both children had got out of the car on their own.

“She’s excited, trying to get things together,” Gonzalez said.

The sheriff said about two to three hours after the mother went into the house, she began looking for her son and went to the car. She found him unresponsive, still buckled in, and called 911.

The sheriff’s office said the child was pronounced dead at the scene.

The mother told the sheriff her son had got out of his car seat by himself before, but the family was using a rental vehicle that may have been unfamiliar to him.

The sheriff’s office said Tuesday that investigators responded to the scene and the case is open. The office said investigators will meet with the district attorney’s office to present their findings.

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