HOULTON — Two of three Massachusetts residents charged in a drug-related killing in northern Maine have pleaded guilty. Tia Ludwick of Leominster pleaded guilty to felony murder and robbery on Thursday as jury selection was about to get started in Houlton. Another defendant, Darin Goulding, pleaded guilty the day before to robbery. A third defendant, Marcus Asante, of Fitchburg, Massachusetts, is awaiting trial in November. The victim, Douglas Morin, was found in his car on Oct.