New Jersey ? A 28-year-old man and a teenage boy were charged Thursday in the execution-style schoolyard killings of three college students and the wounding of another, a crime that has outraged this violence-marred city.

The arrests came within hours of each other, with police taking the 15-year-old boy into custody Wednesday night and the man, Jose Carranza, surrendering to the mayor Thursday.

Authorities have said robbery appeared to be the motive, and robbery charges were brought against the two suspects.

The four victims, friends who were planning to attend Delaware State University this fall, were shot while visiting in a schoolyard not far from their homes Saturday night.

Terrance Aeriel, 18, Dashon Harvey, 20, and Iofemi Hightower, 20, were forced to kneel against a wall and were shot at close range. The fourth victim, 19-year-old Natasha Aeriel, Terrance Aeriel’s sister, survived a wound to her head and is hospitalized.