who can be held liable for faulty repairs from 2 different mechanics I don't know which one caused the problem

Who is liable?: What am I to do when two incidents happened within a short time of each other, and I don't know which one caused the mechanical problem with my car?

A week ago I had my intake manifold gasket replaced. Then yesterday, while my car was parked legally, a neighbor "slid" in to the rear end of my car and then accelerated causing my car to move forward almost 2 feet while still in park. (The police were called and he admitted to sliding in to the car, but not accelerating. We have a witness saying that the neighbor did indeed accelerate, spun his tires, and moved my car nearly 2 feet forward).

An hour later, while driving, my car began to make loud noises and was running really rough. I had it towed to a second mechanic (someone other than the one who replaced the intake manifold gasket) to be assessed. There is now a definite problem with the engine, the damage estimate was $800 at minimum, the car could possibly need a new engine.

I'm trying to figure out who is responsible for this damage. I plan on getting the opinion of the second mechanic, but the first mechanic (who replaced the gasket a week ago) already claims no responsibility for the problem.I have a feeling that the problem is most likely the shoddy work of the first mechanic, but what if it is also possible that the rear-ending caused it? What actions should I take? I don't know who to go after for the repairs my car needs.

If it's possible that the mechanic didn't cause the problem, how can I expect him to be responsible for these repairs? And, if it's possible that the neighbor didn't cause the problem, how can I expect HIM to be responsible for this? I just don't want to be caught with this bill because it is clear that it is ONE of their faults, I just don't know who... - Is this your question? Add additional information