Also of note from this
NBC Boston article: “Those who live nearby say they're surprised they didn't hear any gunshots.”
Depending on how far along they were on the trail, I could see how gunshots wouldn’t be heard, but at the same time the sound of gunshots travels quite far even in heavily wooded areas. When I used to live outside of Seattle, there was a mountain trail I would run that was adjacent to a shooting range and even if I was a mile away from the range I could still hear gunshots quite distinctly.
Perhaps someone who has more experience with firearms can chime in here as to how likely it is gunshots would be heard by those who live nearby. Perhaps a silencer was used?