A Los Angeles-area grandmother and her dog were stabbed to death Thursday by an attacker who was then knocked out by a bystander with a rock, authorities said. 

The woman, identified as Jeanne Edgar, 66, and her dog were killed near a San Dimas park by a man who appeared to be in his early 20s, authorities said. Los Angeles County Sheriff deputies responded to the chaotic scene around 1:30 p.m. amid reports of an assault. 

"The call indicated a woman was screaming and later updated indicating a woman was being stabbed," the department said. 

A woman and her dog were fatally stabbed Thursday by a man near Lone Hill Park in San Dimas, Calif., authorities said.

A woman and her dog were fatally stabbed Thursday by a man near Lone Hill Park in San Dimas, Calif., authorities said. (Google Maps)

Deputies found the woman with multiple stab wounds and she was pronounced dead at the scene, authorities said. The dog was lying next to her and was also pronounced dead. 

Witnesses said the alleged attacker kept yelling that he was going to kill someone. He first went after a man in his 30s who ran away from him, authorities said. The suspect then turned his attention to the woman walking her dog. 

"The suspect grabbed the female victim’s dog by the leash and yanked it free of her grasp, authorities said. "The suspect then stabbed the dog and then the female victim."

The man who fled the suspect earlier spotted the attack on the woman and used a rock to knock him unconscious. 

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"He, at that point, got out of his vehicle, found a rock, threw the rock, hit the suspect in the face, knocked him out and right shortly after that the authorities arrived," one witness told KCBS-TV.  

The suspect, identified by the news outlet as Ricardo Saldivar, was taken into custody. He was taken to a hospital to be treated for head injuries. He is being held on $2 million bond. A motive for the attack has not been determined.