I raked a little bit in my older power point slides from a seminar I once participated in about Terrorism, and I found this quite interesting definition of the term “Terrorism”:

“Terrorism is an anxiety-inspiring method of repeated violent action, employed by (semi-) clandestine individual, group, or state actor, for idiosyncratic, criminal, or political reasons, whereby – in contrast to assassination – the direct targets of violence are not the main targets. The immediate human victims of violence are generally chosen randomly (targets of opportunity) or selectively (representative or symbolic targets) from a larger population, and serve as message generators. Threat – and violence – based communication processes between terrorist (organization), (imperiled) victims, and main target (audiences(s)), turning it into a target of terror, a target of demands, or a target of attention, depending on whether intimidation, coercion, or propaganda is primarily sought.”

(Schmid, Jongman et al. Political terrorism: a new guide to actors, authors, concepts, data bases, theories, and literature. Amsterdam: North Holland, Transaction Books, 1988)

Taking this into account while reading the first link you send me, led me to the conclusion that Mexican drug cartels are technically not a terrorist organization. I believe it is due to their criminal organization and purpose that they fall out of the definition of a classical terrorist group. They do not seek for public attention in order to change the state of mind of the population. They rather try to fight other drug cartels for their goal of supremacy within the drug market. Terrorist groups, as Schmid puts it, strive for an ideological goal and thus choose their targets randomly. As I understand the procedure of drug cartels, they choose their victims with precision (e.g. the FBI agent mentioned in the article) in order to iron out hindrances.