The puppy named Buddy was only five months old.
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The owner wrote on Facebook that Buddy was being transferred to the vet to be neutered and was left in a hot van.
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Pasadena's mayor Jeff Wagner responded with a post of his own saying, "I cannot fathom the heartbreak they must be feeling."
He also wrote he is assigning the case to the police department's public integrity unit and after a thorough investigation, potential disciplinary actions will be rendered.