California began identifying certain vulnerable inmates as needing special protective housing starting in the late 1990s and created so-called Sensitive Needs Yards for those inmates beginning in early 2002.

However, an Associated Press investigation found that the yards have developed their own problems, helping to push California's inmate homicide rate to double the national average.

The AP found that male sex offenders make up about 15 percent of the state's prison population but accounted for nearly 30 percent of the 78 California prison killings reported by corrections officials since 2007.

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WHAT ARE THEY?

Sensitive Needs Yards are separate cell blocks, common areas and outdoor recreation yards set aside for inmates who would likely be harmed if they were in the general population with other prisoners. They include sex offenders; inmates with youthful appearances; inmates who have left prison gangs; have been assaulted; have enemies in the general prison population; have drug or gambling debts; or are former law enforcement or correctional officers.

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WHO IS THERE?

California prisons hold a total of 37,457 inmates labeled as "sensitive needs," nearly 28 percent of the overall inmate population. Forty percent of the sensitive-needs inmates are registered sex offenders.

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WHERE ARE THEY?

California has 47 of the yards scattered within 21 of the state's 34 prisons. All of them are in men's prisons. Women's prisons do not have Sensitive Needs Yards.

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HOW MUCH DO THEY COST?

Prison officials say there is no additional cost, as Sensitive Needs Yards are run like any other housing unit.

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HOW ARE THEY PART OF THE PROBLEM?

Of 11 inmate homicide cases closed by the inspector general's office during the first half of 2014, 10 were of sensitive-needs inmates. Of those 10, eight were killed in their cells. Another sensitive-needs inmate was seriously injured in his cell but survived.

Prison officials have identified nearly 100 gangs within Sensitive Needs Yards, according to Matthew Buechner, a special investigator who trained other corrections officials on problems with prison gangs until he recently retired.

Corrections department spokeswoman Terry Thornton said officials are most concerned about "a handful" of the most active gangs.