Pet death due to sitter negligence - what should I do?

I booked with a http://Rover.com sitter on Saturday, Nov 3rd when my husband and I were out of town attending my cousin's wedding. He had 4-5 five-star ratings and lived nearby, so I felt he would be a good fit. He had text me earlier on the day we planned to pick him up that he would be available any time after 6, but possibly earlier depending on his "clients". I thought it was a little odd that a sitter would be working during the time he was meant to be be watching someone's dog, but I assumed he knew what he was doing and that the dogs were safely inside his house at the very least.

I got a text saying he was home and left to pick up my dog. When I got there he asked me if I had come by and got my dog while he was at the store (?????), then informed me that he couldn't find him anywhere and that he'd (the dog) been sitting on his outside furniture when he'd left for the store. Ensue us looking all over the house, the yard, and soon the entire neighborhood for the next 4 hours in the dark.

The sitter eventually text me saying "found her", then said his friend had found someone on our town's lost and found Facebook page who had posted a picture of our dog's collar. I then waited for said person to get back to me or the sitter's friend with an address. Eventually I was given a phone number. The woman who I called had found him dead on the side of the road and taken his body to her house.

She gave me her address and my husband and I, heartbroken, returned with his body around 9:30 PM. I won't go into how much he meant to us, because that would take a while. But I will say that we loved him beyond words, treated him like our child, and needed the next day or two to recuperate. We even called off work to get our heads around it and bury him.

I should also add that someone also commented on the FB post saying that they had seen our dog earlier that day walking near the road, around 12-1 PM, but that he'd run back in the fence so they assumed he lived there or new the dogs. So this wasn't some miraculous one-time escape, and he'd obviously been left outside most of the time with little to no supervision.

And now I am left with the question of what to do. I feel like at the very least we should be reimbursed for the time he was "watched" and that the sitter never be allowed to use Rover again. Does anyone know what I should do/who I should contact about this?