A 5-year-old boy in Texas died this week after his family left him in a hot car for several hours in triple-digit heat at their home as they prepared for his sibling’s birthday party.

As NBC News reports, the unidentified boy was found in the car somewhere between two to three hours after he was left there. The Harris County Sheriff Ed Gonzalez said the child was pronounced dead at the scene in Houston, where it reached 102 degrees on Monday, and an investigation has been launched.

Authorities said the family informed them that they were preparing for the boy’s sister’s birthday party at the time. His mother was “excited, trying to get things together [for the party]...with the busyness of the activities that they were preparing for, it took them awhile to notice that the child wasn't in the house," Gonzalez added.

The older sibling exited the car on their own, and the mother had “assumed the other child was out also.” The family said they believed the boy knew how to unbuckle himself, but it was a “loaner car” and he may have been unfamiliar with how to get out.