The agency says it will provide details Thursday during a late morning news conference at the Mariposa border crossing in Nogales, Arizona.
Authorities say illicit fentanyl in recent years has become the biggest source of fatal overdoses in the United States.
Mexican traffickers are increasingly smuggling the drug into the United States, mostly hidden in northbound passenger vehicles crossing at ports of entry in the Nogales and San Diego areas.
Law enforcement says the illicit version of the painkiller is now seen mostly as a white powder that can mixed with heroin for an extra kick and as blue pills that are counterfeits of prescription drugs like oxycodone.
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