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."
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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.