Complex sentence: It is believed Fred West killed her to avoid any investigation into Charmaine's whereabouts, and Costello is believed to have been strangled to death by Fred before her extensively mutilated body was buried in Letterbox Field.
Split into simpler sentences: It is believed Fred killed Costello to avoid any investigation into Charmaine's whereabouts. She is believed to have been strangled to death by Fred before her extensively mutilated body was buried in Letterbox Field.