What Do I Do If I Think My Neighbor Is Stealing My Water? Ma

Location: MA

This is a long story, I own the third floor of a triple decker condo and I believe my neighbors who own the second floor have tapped into my pipes and are stealing my water.

I think this because my water bill went from 3-6 units of water used in a quarter to 21-28 units of water in a quarter. Normally I would assume a leak is the cause of this but the folks on the second floor had their water shut off by the water company back in June of last year. I know about the water being shut off because in June they texted asking me if I had a plumber over recently because they didn't have any water. The water company also confirmed this information during a site visit last week when they looked at all the water meters in the basement and noticed that the water for the second floor is shut off at the street and was shut off in June.

The reason why I don't think my bill jumped duke to a leak is this past Thursday I turned off the water to my unit before I left for work. My spouse also left for work at the same time, so there was nobody home during the day. When I turned off the water at 7am I took a picture of the meter. When I returned home that evening I went into the basement and found that my water was turned back on and the meter had spun. My spouse and I are the only people who live in our unit and a leak couldn't turn the water back on.

My question is what do I do next and who do I go to? I am in the process of getting a camera to point at my water meter so I can get a video of whoever is turning my water back on and causing the meter to spin when we are not home, but I don't know what to do.

TL;DR- Water bill is 7x higher, and after turning off the water someone unknown to me turned it back on and used water while no one was home. I suspect a neighbor is stealing utilities. What do I do?