david duane watson , a 46-year-old tucson firefighter , was arrested without incident saturday on charges he killed his ex-wife linda watson , 35 , in what detectives say motivated by a custody dispute .
police say the dispute continued after watson 's murder as her mother marilyn cox , 63 , tried to obtain visitation rights .
three years after her daughter went , cox and her neighbor renee farnsworth , 53 , were shot dead in the driveway of linda watson 's home .