The number is now five Zetas that have been detained in Nuevo León. Governor Rodrigo Medina says that the attack was "directed at the casino and not the civilian population". I'm not sure if that means the deaths were an accident, according to authorities, or if he's just saying that to reassure the locals. There have been lots of reports from witnesses, however, that say that the men behind the attack were screaming at everyone to get out when they started to light the casino on fire, which seems to support Medina's version.
Anyway, this again demonstrates that when the security agencies are properly motivated, they are as capable of tracking down the bad guys as one would hope. The problem is that, too often, only something truly horrific seems to serve as proper motivation.