What important information does David tell his parents at the of part two? What are the implications of what he said?

David tells his parents that he saw his Uncle Frank at their house earlier in the afternoon, before Marie was found dead. David was out riding his bike with his friends, and stopped at the McAuleys’ outhouse to use the bathroom. When sitting in the outhouse, he saw someone looking like Frank cutting across their backyard, heading towards town. The implications are that Frank had something to do with Marie's death.