Detective Mackey was the poilceman responsible for the investigation of the Joe St. George case. He rightfully believed, that Dolores Claibourne had killed her husband, but couldn´t prove it. When Vera Donovan died eighteen years after that he automatically thought, that Dolores murdered her. Tht´s why he investigated her and called her daughter as a part of a plan to try to nail her through her, because he knew she suspected, that she murdered her father, too. His purpose was to arrest her for both murders. His investigation is put to an end, when Selena decided to protect her mother for what she did to her father realising that she lost her right to pursue the matter, because of the way she handled it and because she realised, that she did it to save her from the sexual advances of her father. He told him in front of everyone at the hearing, that he also lost the right to pursue the matter, because he had behaved the same way to the extreme of not realising, that Vera Donovan´s death was a suicide, something she realised after realising the truth about what happened to her father and why.
He has no choice, but to close the investigation and let Dolores go.