I don't know why they needed to do this. They have the corpse of the shooter, just press it's thumb on the phone to unlock it.
Ignoring moral and sanitary considerations of accessing a dead body, it just won't work. Most modern phones use capacitive sensing during the process. Once tissue is dead, it loses its electrical charge and will fail to activate the fingerprint sensor, making it impossible to unlock the phone this way.
More phones should check if the thumb has a heartbeat first.