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Authorities say a 5-year-old boy left in a vehicle has died after temperatures rose above 100 degrees as he prepared for his mother’s birthday party in the Houston area.

Harris County Sheriff Ed Gonzalez said the boy died Monday while his family was preparing 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 thought the two children had gotten out of the car by themselves.

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

Two or three hours after the mother went home, she began searching for her son and drove to the car, Sharif said. She found him unresponsive, still in custody, and called 911.

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A mother in Texas has died after leaving her baby in a car while preparing for a party.

A mother in Texas has died after leaving her baby in a car while preparing for a party.

The sheriff’s office said the child died at the scene.

The mother told Sharif that her son had previously got himself out of his car seat, but the family was using a rental vehicle unknown to him.

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