a 41-year-old driver hit and killed a 2-year-old girl sunday evening when the toddler ran front of his van in a milwaukee neighborhood .
the driver , archie brown jr. , stopped at the scene .
moments later , someone from the home where the toddler lived opened fire - shooting brown dead and fatally wounding a 15-year-old .
police are investigating whether the shooting was revenge for the fatal traffic accident .
the gunman responsible has not been arrested .
brown was a father of four with a young daughter of his own .