Without a witness, a murder weapon, it's going to be a circumstantial case, (which happens way more than most realize), they are going to put him in the home with DNA (that is truly amazing work if they did pull a DNA sample out of there, that fast), put him near the crime with the car, cell phone data, they are going to establish google searches, online posts, maybe, social media?
They have to connect him to the victims some way. There has to be some real connection. These girls probably never knew he existed, or barely, but somehow he was angry with them. There's a reason why them. There's hundreds of girls like that. and dozens in that immediate area. There's a connection.
We know he had no accomplices, and probably was too smart to tell anyone, off line.