Jamie Cullen is the son of Melanie Cullen, who killed Jamie's father Edward and his mistress when Edward was planning to divorce Melanie. Melanie manipulated Jamie into providing a fake alibi, but she was arrested when that fell through.
Jamie was pushed to testify on the stand, but he perjured himself by saying Melanie was aloud entry to scene where the murders happened, not that she broke in as was discovered. The prosecution was afraid of Melanie being acquitted, so they pleaded her down to manslaughter. (L&O: "The Wages of Love")