Walsall schoolgirl Mylee Billingham died from a single stab wound to the chest, a post-mortem examination has found.
West Midlands Police said a 54-year-old man, believed to be Mylee's father Bill Billingham, was arrested on suspicion of murder on Tuesday and will be interviewed when he is deemed medically fit.
The father of eight-year-old Mylee was originally detained on suspicion of attempted murder on Saturday as emergency services called to a property in Brownhills tried to save her.
In a short statement, the West Midlands force said: "A post-mortem examination took place on Monday and the results confirm that Mylee died of a single stab wound to the chest.
"The 54-year-old man remains in a serious but stable condition in hospital. Police continue to support Mylee's family at this tragic time."
Officers were called to Valley View in Brownhils, where Billingham was found with wounds to his stomach at about 9.15pm.