Eleven Texas prisoners have been killed this year inside the state's prisons the highest number in at least 15 years.

Last year, just three inmates were killed and this year's total is the highest since 1997, when 10 were killed according to the Austin American Statesman.

Officials said the increase is puzzling since overall violence, sexual assaults and the number of weapons found in prisons statewide, have all declined in 2012.

Investigations into individual cases show that the homicides appear to be random acts of spontaneous violence between cellmates with no apparent pattern.