Story highlights "I am in shock that someone would treat a dog like this," a witness says

An advocacy group is offering a reward for information on the dog and owner

(CNN) Two men in a pickup truck abandoned a dog at a Louisiana gas station and drove off, leaving the animal desperately chasing after them, an animal advocacy group says.

"The first man told the dog to: 'Go! Go over there!' The man in the back of the pick-up proceeded to pull up the tail-gate of the truck," Lorie Hollis, a witness, said in a statement Monday from the Humane Society of Louisiana . "Appearing confused, the dog jumped up and put his paws on top of the tail-gate. The truck sped away, leaving the dog behind"

Hollis said she got in her car and went after the truck and the dog, described as a black German Shepherd-Lab mix with a small patch of brown under its neck.

"The truck crossed over two lanes of traffic, and still the dog followed," she said.

Eventually she lost track of the vehicle and its canine pursuer.