I think you could probably just make an anonymous tip.

Something like 6 years ago I was visiting back home for a few days and was having a night cap at a friends place after going out. We didn't have weed so me and a couple friends went to pick some up. On the way back a car in front of us got T-boned when someone sped through a red light. We all got out to check if everyone was alright, which thankfully they were but understandably shook up (driver who ran a red light was also clearly drunk). We called the police and gave a statement over the phone, stayed with the girl who got t-boned a little longer until others showed up who could stay and then bounced before the police could get there just in case.

So technically you could totally call and then leave; surely they'd preferred you stayed but there's not much they can do to stop you. Obviously, though, this was in the 50s so quite different circumstances but nonetheless, he could have used a payphone nearby, left the information and left.