Doug Sovern, CBS San Francisco, January 14, 2013

Two groups are at war with each other on the streets of Oakland, after the murder of woman last summer, and the subsequent feud is responsible for "90 percent" of the violence in the city since then, Oakland Police Chief Howard Jordan said Monday.

City officials held a news conference Monday to discuss the alarming wave of shootings in the city over the weekend. Four people were killed Friday and 11 more were wounded since then.

Oakland Mayor Jean Quan, Police Chief Jordan and City Administrator Deanna Santana joined other local leaders in East Oakland to address a disturbing string of shootings in a city that has been plagued by violence and a soaring murder rate.