ceiling leaks when upper tenant takes shower...why?

[starfish11040]

There are two apartments in my building almost identical (same bathroom layout). A few months ago a leak started from the ceiling of the lower bathroom. It is from the ceiling directly over the sink. It seemed to coincide with the tenant in the upper bathroom taking a shower.
We discovered that some tiles were loose in that shower wall, and replaced all of them. That was a couple of months ago and I thought the problen was solved. But it has now recurred. (It may have been happening but unnoticed when the lower tenant wasn't home). It is worse than ever and it does occur when the upper tenant is taking a shower. I do not believe the problem is the tiles.
What are the possible causes?

 

[majakdragon]

The drain piping below the tub/shower may be leaking. You will probably need to remove a section of the ceiling to investigate.

 

[phishfood]

Unfortunately, I think that majakdragon has about the only answer. It will be really hard to pin down the problem without doing this.

 

[CleveJeff]

Before tearing out the ceiling, it might be a good idea to check the screws on any fixtures attached to the tile wall. This would include soap dishes as well as handicap bars. Be sure to verify that any screw sinks used are still tight in the tile. If the shower user points the water flow at these leaking screws, quite large amounts of water can flood the ceiling below.