What is the test for finding a prison official liable for failing to protect an inmate from physical abuse?

To establish a violation of a prison official's duty to take reasonable steps to protect inmates from physical abuse, the prisoner must establish that prison officials were "deliberately indifferent" to serious threats to the inmate's safety. Farmer, 511 U.S. at 834. "Mere negligence is not sufficient to establish liability." Frost v. Agnos, 152 F.3d 1124, 1128 (9th Cir. 1998). Rather, a plaintiff must set forth facts to show that a defendant knew of, but disregarded, an excessive risk to inmate safety. Farmer, 511 U.S. at 837. That is, "the official must both be aware of facts from which the inference could be drawn that a substantial risk of serious harm exists, and he must also draw the inference." Id.