Walls and Ceilings - Water leaking in between walls

It rained really hard here in San Francisco in the past few days and I noticed my wall(near the front of the house where it touches the neighbor's house)is leaking water in from the space in between my house and the neighbor's house. I went outside to see if there were any obvious signs of cracks or openings that can cause rain water to leak inside. As I probe the area where my house and the neighbor's touch outside I noticed that there was a crack along the border of my house and the neighbor's house and as I pressed on the neighbor's side, the wood was so "dry-rotted" the it fell inside exposing the space in between my garage wall and the neighbor's wall. And as I shined a flashlight in there, I noticed water is leaking from above. What/who can I contact to determine where the leak is coming from. Since the neighbor's side has rotted wood that upon pressing on it slightly it breaks apart, I can only assume it is their responsibilty that I have water seeping into my garage from the wall that faces their wall, and water seems to be coming down to where it comes out between the wood and concrete foundation in my garage. Any suggestions can help. I will take a picture and add it here for people to see.