Authorities on the scene of a fatal shooting on the 9400 block of Hillside Street in Oakland on January 11, 2013. It was the fourth homicide in a six-hour span. (CBS) Authorities on the scene of a fatal shooting on the 9400 block of Hillside Street in Oakland on January 11, 2013. It was the fourth homicide in a six-hour span. (CBS)

OAKLAND (KCBS) — 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. Police said one suspect is in custody.

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.

There were four separate murders within six hours Friday afternoon and evening, with three men and a 17-year-old killed in what police believe were related, retaliatory shootings.

Three people were wounded Saturday afternoon in a shooting on Harmon Street. There was another shooting later that night, followed by two more on Sunday.