Unable to Press Charges

According to the PGR, three quarters of those arrested for drug crimes in Mexico since the beginning of Calderón's term have been released without facing charges. In Chihuahua, where close to 6,000 people have been arrested, half have been released. In Jalisco, where the total number is almost eight times higher, 90 percent have been released. I'd like a little more explanation as to why: is this the product of mass arrests, or of lack of capacity to process the offenders, or of judicial norms that tilt the playing field in favor of the defendant in certain cases, or what? I also wonder what the corresponding stat would be in the US.