Under the doctrine of "respondeat superior," an employer is often responsible for the wrongful acts of his or her employees. There are limitiations--an employer is often NOT responsible for acts which are completely unforeseeable and/or outside the scope of employment. Since the workers were not hired to steal, the theft may be something for which the employer cannot be held liable--it's not foreseeable or actions within the scope of employment--unless the employer was liable in some way. For example, if the employer did not supervise employees properly; or did a poor job of hiring and screening; or retaining and used someone whom they knew had stolen previously; in those cases, the employer (the contractor) would be liable. So the worker who stole can be sued by you--anyone who steals may be sued directly. And you may be able to sue the contractor who employed him if the contractor was somehow at faut or negligent, and it'd probably be worth it, if you  bring a lawsuit, to name the contractor and try to hold him or her accountable, too.