Remember what Lisa said when she gave you the names of the two girls?
She said she referred them to Gavin, and you shouldn't come back until you could prove what happened to them after that.
Well, you can prove it now. Go back to Lisa and give her the information.
You don't have to go back to Lisa until you've finished investigating both girls, but you'll tell her about them separately, so you might as well go there after each is done.