UPDATE: John Zaremba reports [2] the victim's condition has been upgraded from critical to serious. NECN reports [3] he was walking to choir practice at his church.
Stanley Staco [4] reports the teen was shot in the stomach around 7:10 p.m. at Humboldt Avenue and Homestead Street [5].
The homicide unit is called in whenever a victim's condition is serious enough that there's a chance he could die.
Links:
[1] http://www.universalhub.com/users/adamg
[2] http://twitter.com/johnzaremba/statuses/289920402178859008
[3] http://www.necn.com/01/11/13/13-year-old-choir-boy-shot-in-Boston/landing.html?blockID=822691&feedID=11106
[4] http://twitter.com/stacos
[5] https://maps.google.com/maps?q=humboldt+ave.+and+homestead+ave.,+boston,+ma&hl=en&ll=42.31191,-71.090512&spn=0.016185,0.033174&sll=42.381897,-71.056673&sspn=0.008084,0.016587&hnear=Humboldt+Ave+%26+Homestead+St,+Boston,+Suffolk,+Massachusetts+02121&t=m&z=15
[6] http://www.universalhub.com/roxbury
[7] http://www.universalhub.com/crime/index.html
[8] http://www.universalhub.com/taxonomy/term/4047