The reporter on BBC1 said they are not allowed to give full details.
The man says he did not murder her, but was probably responsible for her death.
One could suppose, for example, that you kidnap someone, take them to the middle of a huge remote forest, and leave them there alive. They may die if they cant find there way out the forest.
So you have not murdered them as such, but are responsible for their death.
And if it is a huge forest (or other remote area) maybe he does not know where she is.