LAREDO — Leads seemed few in the days after singer Valentin Elizalde was gunned down in Reynosa, Mexico, last weekend — no arrests made, no suspects named. But in cyberspace, clues were plentiful. Nicknamed "The Golden Rooster," Elizalde, 27, was well known in Northern Mexico for his brass-based traditional "banda" music and for his lyrics, some of which glorified the drug trade. Three months before his death, someone posted a photo slideshow on the popular video-sharing Website YouTube.com depicting a succession of bullet-riddled bodies, with Elizalde's song "A Mis Enemigos" — To My Enemies — as the soundtrack. The gory...