who is at fault

What is the name of your state? texas
I pulled into a parking space at a gas station. As soon as my car stopped, I killed it and my passenger and I proceeded to exit the car. My passenger looked out the window and there was an empty space beside him. As he opened the door at normal pace about halfway, a collision occured suddenly with a truck pulling into that previously empty space. we could not see this coming because the truck originated from behind us. the contact was against the edge of my door and just above the peak of the truck's front left wheel well. both vehicles are approximately each 1 foot from the divider line. the truck received minimal damage but my car door is crumpled a bit due to the force of the moving truck.
he says he waited for us to get and then slowly crept into the space and the door flew open suddenly and there was nothing he could do. He couldn't have waited, we began to exit the car as soon as it came to a stop. we believe he was traveling faster than he should have been and that's why he wasn't there one moment and was there the next. fact is we were stopped and using caution. he was moving and plowed into my door. the police came and made a report. they said it wasn't their job to assign blame. now the guy has made a claim against my insurance. It seems like everybody is leaning in this guys favor...i am thinking about filing back. who will the insurance companies find at fault?

[JETX]

In my opinion, the obligation is on you to make sure that it is safe to open your door. And this includes looking behind you to see if a vehicle is entering. Simply, the assumption is that a vehicle is pulling into that empty spot.... until you are convinced it isn't. Same principle as opening a 'street-side' door into a traffic lane (when parked at the curb).