He must have seen her in the yard.
Could have
Or seen her twitter feed/instagram/facebook/snap chat/whatsapp/or whatever as well
Heck, he could have created a fake 13-15 year old boy account and communicated with her that way as well. He’s 21 not 61.
We don’t know yet
This is one case where the death penalty should be swift. He's a serial murderer and pedophile so he would do it again.