ST. LOUIS (AP) - Mexican drug cartels are quietly filling the void in the nation's drug market created by the long effort to crack down on American-made methamphetamine, flooding U.S. cities with cheap, potent meth from factorylike "superlabs."

Although Mexican meth is not new to the U.S. drug trade, it now accounts for as much as 80 percent of the meth sold here, according to the Drug Enforcement Administration. And it is as much as 90 percent pure.

The cartels are expanding into the U.S. meth market just as they did with heroin: developing an inexpensive, highly addictive form of the drug and sending it through the same pipeline already used to funnel marijuana and cocaine, authorities said.

Seizures of meth along the Southwest border have more than quadrupled over the last several years.