Three other siblings, including one who was bleeding from puncture wounds, escaped by climbing through windows and running through fields to a neighbor's house, where one called 911.
"There's somebody in my house who I don't know," 14-year-old Jessica Lynn Carpenter told the dispatcher. "(He's) stabbing my brother and sister with a pitchfork. You have to careful â€“ he's going to kill them."
When deputies arrived at the house in this rural community about 60 miles north of Fresno, the stranger charged at them with the pitchfork, authorities said.
"He was pointing it at them and going right after them. He wouldn't stop," Assistant Sheriff Henry Strength said. "They were hollering at him to stop, but he wouldn't stop. Finally they had to shoot him."
The man's identity was not immediately known, but there appeared to be no connection between him and the family, Strength said.
"He came from nowhere. Right now we have no motive. We have no reason why this thing even happened," Strength said.
Ashley and John Carpenter were killed in their beds. Anna Marie, 13, was hospitalized and in satisfactory condition Thursday.
The children's parents were not at home at the time, the Merced Sun-Star reported.
Jessica told authorities she had just woken up and was walking out of her bedroom when she saw the man in the house. He had closed all the windows and pushed the furniture against the walls.
The girl told relatives she ran back to her room, locked the door and tried to phone for help but the line was dead. She escaped out her window and ran barefoot through the fields until she found a neighbor who was at home.
"It's worse than any horror you could imagine," said Priscilla Hilton, 20, the victims' cousin.