A prisoner was stabbed to death at Folsom Prison, the 172nd stabbing at the institution this year and the fourth fatality. The victim was identified as Michael Keith, 30, committed from Solano County for a three-year and four-month term on drug, forgery and escape charges. Prison spokesman Lt. T. J. Smith said Keith died of one stab wound under his rib cage as he was exercising in the general population exercise yard. Officers said Wayne Richardson, 29, serving 31 years to life for murder in Santa Clara County, was placed in "administrative segregation" pending investigation. The weapon recovered at the scene was believed to have been made from a piece of bed frame. Smith said the motive is not known.