Boston PD via the police scanner confirms an arrest
http://www.broadcastify.com/listen/feed/6254/web
That site said no arrests have been made.
I just heard from your link that the media reports have not been confirmed and as far as he knows (whoever the LE talking) there was no arrest. Now, maybe that’s a CYA. Who knows.
Boston PD Scanner says “NO ARREST”, and they should “Remain vigilant in their mission.”