Ruben "Nite Owl" Castro, 46, a leader of the Mexican Mafia prison gang who authorities said controlled the two cliques of the 18th Street gang targeted in the indictment, the Shatto Park Locos and the Hoover Locos. Castro allegedly ran the enterprise from the Administrative Maximum facility in Florence, Colo., where he is serving a life term after being convicted on similar gang racketeering charges in 1997. With the highest level of security in the nation, the Supermax houses Unabomber Theodore Kaczynski, shoe-bomber Richard Reid and 9/11 plotter Zacarias Moussaoui. Under constant surveillance, Castro had to use cryptic phone conversations and hidden messages to get his directives to his deputies on the streets, who are described in court papers as "shot callers." At least two letters seized by the FBI were scrawled in thick pencil marks, which when erased revealed tiny messages written with a fine-point pen. "We have a couple letters with significant messages...