Tribal violence in the city of Warri continues, with at least four people killed when an Ijaw town was attacked. The fighting has been complicated by the involvement of criminal gangs that specialize in stealing crude oil. This has been going for almost as long as oil has been pumped in the Niger river delta. But in the last few years, the tribe based gangs have gotten more ambitious and started to trespass on territory (parts of pipelines easy to tap into and oil storage facilities) claimed by other tribes.