Two shot in Codman Square

If the person was shot on the bus does the Transit Police investigate or all shootings in the city fall under the jurisdiction of the Boston police. The other night a person was shot and killed on the road at the Gallagher terminal in Lowell which is owned by the MBTA but the Lowell police are investigating. How do these investigations work and who decides who is in charge?

This confuses me. A teenager was killed on kenwood st not too long ago and now a month later reports of gunfire on the same street. Police need to be actively posted in these troubled areas and not just the day after someone is killed.