The hunger strikers, who have been refusing meals since July 8, are demanding an end to indefinite solitary confinement because they believe it constitutes cruel and unusual punishment.
The letter, organized by the National Religious Campaign Against Torture, describes solitary confinement as "a practice where prisoners are confined to a small, windowless cell, without sunlight, fresh air, meaningful human contact or constructive activity for many years, even decades."
Our various faith traditions hold in common a belief in the dignity and worth of each human person. Prolonged and indefinite solitary confinement violates basic religious values of justice, compassion, and healing.
Steinem and Jackson are long-time civil rights crusaders. But Raitt's interactions with prisoners during a performance at San Quentin "made a profound impact" on her, while Jay Leno's wife, Mavis, was inspired to join the cause after a conversation with UCLA psychiatry professor Susan Smalley.
"I was appalled at this unlimited, indiscriminate use [of isolation] by prison administrators, so I rallied my network," Smalley said.
The hunger strike started with as many as 30,000 inmates in early July, though numbers dwindled as time went on. The remaining group includes convicted murderer Todd Ashker, who earned a paralegal degree while in prison.
"There's a core group of us who are committed to taking this all the way to the death, if necessary," he said in a video made for his cause. "None of us want to do this, but we feel like we have no other option."
Notable deaths of 2014 [PHOTOS]