Oakland police said they believe suspect Mario Toliver Jr. of Oakland may still be carrying the weapon involved in Thursday's shooting.
"We have a serious concern, because we have a 14-year-old, who we consider armed and dangerous," said Officer Johnna Watson, a police spokeswoman. "We have not recovered a weapon from the scene so we believe he may still be in possession of that weapon, in this case, a handgun."
Police have not released a motive in the shooting. But relatives have said the shooting occurred after the two siblings got into an argument over laundry at the family's apartment in Oakland's Chinatown.
Relatives have identified the victim as the boy's sister, Justice Toliver, and said she has a 2-year-old daughter.
"They got into some type of altercation," Gregory Stewart, 25, a cousin, told the Oakland Tribune on Thursday. "Whatever it was about it shouldn't get that serious to pick up a gun and kill his sister."
Mario Toliver, Sr., who was not at home at the time of the shooting, pleaded with his son to turn himself in.
"Just call," the father said. "Please son, turn yourself in, or please call your daddy so we can turn yourself in. I love you and I don't want nothing to happen to you."