If you can prove by a "preponderance of the evidence", such as security camera footage, police & arrest reports, the disposition of any criminal cases, etc. that the damage was done by the other owner's squatters, you can likely recover your losses from him in a lawsuit, since it would be negligent, or unreasonably careless, for a building owner to not maintain his property and allow it to be used by squatters and criminals. The issue is showing with some evidence that the problems came from his units; it's not enough that it's logical that was the source, but rather you have to have proof.