It depends on whether the management office was reasonably careful or not in giving him the key. They are not liable for this criminal act itself--one person (or entity, like an LLC or corporation) is not liable, or responsible, for the criminal acts of other persons unless they knowingly participated in, assisted, etc. those acts. So their only liability would be for their act in giving him the key.You say that you had submitted an application to remove him, but the revised lease was not yet signed. That means that legally, he was still an occupant; therefore, you son was legally entitled to enter. If it was your son then, the landlord/management company would not be liable--they allowed iin a person who was allowed in.Or if it was not your son, but he had good forged ID that would pass a reasonable inspection, then the management company would not be liable either if they did look at the ID to verify who it was--they are not liable if they took reasonable steps to ensure security, but were tricked by someone.But if it was not your son and the person did not have ID showing him as your son, or he did but no reasonable person would have been fooled (e.g. ethnicity, age, or appearance, etc. did not match), then the managment company would likely be liable. In this instance, they did not exercise reasonable care to secure entry and protect their tenants.