California prison inmates housed in the state's highest-security prison have sent an open letter to Gov. Jerry Brown, threatening hunger strikes and work stoppages if the state does not limit the length of time prisoners can be held in isolation cells.

The undated letter, signed by four prisoners housed in segregation at Pelican Bay State Prison, contends California prison officials failed to deliver on promises made to end a series of prison hunger strikes that involved as many as 6,500 inmates in 2011. Giving a July 8 deadline, the inmates ask for an end to indefinite holding of prisoners in Security Housing Units, where they are isolated from other inmates, denied privileges and allowed out of the cell 90 minutes a day.

The state uses Security Housing Units to segregate inmates who are believed to be leaders of prison gangs or pose other security dangers. In 2012, California began reviewing inmates housed in segregation for inclusion in a five-year "step down" program that moves them back into the general prison population. Previously, the state required SHU prisoners to confess and provide incriminating evidence of gang activities in order to get out.

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