Cynthia Thomas is a young chemist and the mistress of physicist Dr. Edward Manning. The affair caused Manning to separate from his wife Florence, who drew out the divorce proceedings in retaliation. Cynthia was enamored with Manning and more spiteful against Florence, as she and manning were hoping to marry after the divorce.
When Florence was killed by a letter bomb, which turned the letter opener she used into shrapnel that killed her, Cynthia was mentioned by Florence's relatives. She herself was questioned, and although she and Manning were cleared of suspicion, it was revealed she saw the real bomber, Max Weiss, arguing outside Florence's apartment with Manning. Manning was the real target, but since he moved out, Florence was killed by accident.
Weiss' lawyer, Shelly Conners, successfully got Cynthia's ID of Weiss thrown out, so Manning had to testify that Weiss tried to kill him because Manning stole Weiss' thesis and covered it up by ruining Weiss' career. Weiss was convicted and imprisoned, and it's unknown what happened to Manning's and Cynthia's relationship. (L&O: "Big Bang")