The idea of thought behind who taught them is dictated by patterns and trends. In other words, who else cut off heads and use car bombs? Let’s start with the obvious first: the vast majority of Islamic terrorist groups. Therefore, one could easily postulate that the Mexican cartels are either copying or incorporating the techniques of Islamic terrorist groups or they are being literally taught these varying terrorist methods by actual Islamic terrorist groups. This then leads to the various reports inb recent years citing the growing convergence between international criminal groups/organizations and varying Islamic terrorist groups, specifically those reports directly aimed at the convergence between the two largest Mexican drug cartels and Islamic terrorist groups.
Example:
http://www.conservativecrusader.com/articles/al-qaeda-and-farc-the-unholy-alliance
http://www.washingtontimes.com/news/2009/mar/27/hezbollah-uses-mexican-drug-routes-into-us/
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I've always assumed there was at least some islamic involvement in the mexican drug trade. After all, heroin and hash from Afghanistan and Turkey is distributed in the U.S. and some of it comes through the mexican border. I'm certain people from the aforementioned countries are "on the ground" in mexico competing for markets for their products... using whatever means necessary.
It's a win-win for them: make money, kill infidels.