Here's a story with a bit more info on what exactly happened: www.examiner.com/a-1163277~Bus_stop_murder_brings__outrage___tears_in_Baltimore.html

from that article:
At about 8 p.m., Hallback had just left a meeting of the Algebra Project, a national mathematics literacy campaign aimed at helping poor students, and was standing at the bus stop with two friends. A man at the bus stop pulled out a gun and demanded their money and cell phones, police said.

The robber ordered them into a back alley and told them to lie face-down on the ground. Then, police said, he shot Hallback in the head.