Irwin , 19 , was unarmed when an officer killed him , police chief says . Officer has been involved in a fatal shooting on duty in 2007 , chief says . Activists hit the streets and take to Twitter to vent anger over the shooting .
Tony Robinson , 19 , was unarmed when an officer killed him , police chief says . Officer has been involved in a fatal shooting on duty in 2007 , chief says . Activists hit the streets and take to Twitter to vent anger over the shooting .