MEXICO CITY — Mexican cartels are increasingly going "old school" to keep supplying America with methamphetamine despite an ingredient squeeze. Some gangs have responded to a Mexican crackdown on their meth chemical of choice – pseudoephedrine – by reviving a production method so old, it was used by U.S. motorcycle gangs and bathtub chemists in the 1970s and '80s, recent seizures show. The re-emergence of the "P2P method" demonstrates how frustrating it is to crack down on a synthetic drug that – unlike cocaine, heroin and marijuana – comes from recipes of chemical ingredients, known as "precursors," instead of a plant. When police succeed in cutting off the supply of one precursor, traffickers move on to or make another. "Chemical restrictions are like squeezing mud, the stuff just comes out between your fingers," said Steve Preisler, who wrote the "Secrets of Methamphetamine Manufacture" under the nom de plume Uncle Fester and is considered the father of modern meth-making.