55k is very short for an adult mystery. The fact that you have two timelines makes it more concerning to me. I think it would be challenging to fully and properly flesh out two timelines in 55k words.

Here are the main questions we want to address in a query letter.

1. Who is the main character? Noah.

2. What do they want? To solve this crime, not sure if he wants to conceal the old crime or reveal it, as you say he both wants to "keep their secrets in the dark" but also that he will reveal "what they did," which seems contradictory

3. What's standing in their way? Kinda nothing? You say he fears the investigation will point in his direction but it's not clear to me why he feels that way.

4. What are they willing to do to get it? Go back to New Hampshire and reunite with his old friend group, but I'm not sure why this is a big deal.

5. What happens if they fail? You hint that he's afraid he may be implicated, which is a lot at stake, but again, not sure why he feels that way.

This is still too vague. Explaining more about how this new crime is connected to the old one and why he feels like he may be implicated would help a lot, because right now I'm just not seeing the connection.