A witness saw a man exit an SUV and place something in a sewer in the 16400 block of Phoebe Avenue on Sunday afternoon.
Moments later, the witness heard two gunshots. The man then got back inside a white Jeep Grand Cherokee with black rims. The witness said the vehicle was driven away from the scene by a woman.
The witness then found a dead terrier mix laying on a dog bed.
Investigators believe the man and the woman are responsible for shooting and killing the dog.
"It's very disturbing," said sheriff's Detective Mark Christiansen. "We see a lot of stuff and this one hit home, especially being the dog person that I am."
The incident was captured on surveillance video, which authorities later released to the public in hopes of identifying the perpetrators.
"There's ways to properly handle old, sick, injured dogs," Christiansen said, "and this isn't the way to do it."
Detectives are asking anyone with information to contact the sheriff's Norwalk station at (562) 466-5419. Those wishing to remain anonymous can call (800) 222-8477.