Employment laws differ from state to state. In the state of North Carolina, a previous employer can NOT release negative information on the phone or in writing to a prospective employer unless they have documentary proof of the negative information. Otherwise, they can be sued by the employee. That is why in North Carolina when companies call to check on references, most previous employers will provide ONLY the dates the employee worked, the job title and sometimes the salary or pay bracket, but no other information.
Check with the employment security commission in your particular state to see what type of information previous employers can release and under which conditions.

