The choking doberman

A couple returned from work, when they found their doberman Sally choking on an object. Fearing for their dog's life, the couple rushed Sally to the nearest vet. When they arrived, the vet immediately took Sally to the emergency room. Exhausted by their day, the couple decided to return home, knowing that Sally was in good hands.
An hour later,the vet successfully extracted the object that causing so much trouble to Sally. When he realized what it was, he immediately called Sally's owners. He tried three times saying the couple had to get out of the house! They were in danger! Since the couple never answered, the vet called the police.
When the cops arrived, it was to late. Sally's owners were dead... Luckily, the cops caught the murderer. What was the biggest evidence? Lets say the killer will need some medical attention to sow back his finger.
Category:Scares
Category:Legend