Mexican cartels are flooding the U.S. with fentanyl and are to blame for tens of thousands of American deaths last year, DEA Administrator Anne Milgram said. The U.S. is "losing a generation" to the drug.

Minutes correspondent Bill Whitaker breaks down how fentanyl has been spreading through the U.S., quickly becoming America's worst drug crisis. Whitaker visited the border with Mexico to see how fentanyl is being brought into the country.

The Drug Enforcement Administration (DEA) plays an essential role in the fight against fentanyl. When agents seize drugs, the evidence gets sent to a DEA lab for testing. The agency reports more than 70% of illicit drugs seized are laced with fentanyl.