Eleven Texas prisoners have been killed this year inside the state's prisons -- the highest number since 10 were slain in 1997, despite an overall decline in violence among the prison population statewide. According to sources, last year just three inmates were killed. Officials say the increase is puzzling since overall violence, as well as sexual assaults and the number of weapons found in Texas prisons, have all declined in 2012. Investigations into individual cases have shown the homicides appear to be random acts of spontaneous violence between cellmates with no apparent pattern. Advocacy groups say more should be done to prevent prisoner killings.