It could be a rat killing them (by simply biting them) and then your adult chickens seeing/smelling the blood & pecking them (after the chicks are dead) making them look as if you have one vicious predator.
This is what happened with my chicks. I never, ever saw the adults picking on the chicks. Started finding dead chicks....got rid of the adults...only to find out....IT WASN'T THE ADULTS...it was a rat! A big rat biting them on side of neck, leaving them for dead. Apparantly, my adult chickens had pecked my dead chicks......