can you sue a district attorney for libel?

Generally, the answer is no.  If the district attorney is performing duties that are generally within the scope of his job as a district attorney, then he has immunity.  That being said, if he or she were acting outside the course and scope of duties as the district attorney, then there could be a case. Generally, libel and slander cases are very difficult.  You have to prove that the published information was false, then you have to quantify damages.  There are several other factors that come into play when the defendant is a governmental worker.