The following year , Harmon left the show after three seasons ( with Abbie Carmichael written off as being called on to serve the U.S. Attorney 's office ) and was replaced by Elisabeth Röhm as Assistant District Attorney Serena Southerlyn . The year after that , Wiest left the show after two seasons and was replaced by retiring U.S. Senator Fred Thompson as District Attorney Arthur Branch , whose character was conceived of as being much more right-leaning than his predecessors in the DA 's office , and was a direct reaction to the September 11 attacks . No mention was made on the show of what happened to Nora Lewin , though producers said her character was only supposed to be an interim DA
September 11 attacks
September 11 attacks
Law & Order