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The US Drug Enforcement Administration, or DEA, said cartels and other criminal groups in Mexico have become the primary source of fentanyl in the US. Some Republican lawmakers have linked the rising fentanyl trade with illegal immigration, claiming that the administration of President Joe Biden has lost border control with Mexico.

"It's your fault," Representative Andy Ogles of Tennessee shouted to Biden as he called for an end to the fentanyl crisis in the US during the president's State of the Union address in February.

"For him to stand there with a straight face, and tell us that he has a solution, when with a stroke of a pen he could have shut down the border… that offends me," Ogles said, linking overdose deaths to drug trafficking on the US border with Mexico. "He has not done a damn thing about it."

Meanwhile, Mexican President Andres Manuel Lopez Obrador rebuked US politicians in March. He said fentanyl is not Mexico's problem, but the US', adding that the US should use family values to fight drug addiction.

"Why don't they (the US) take care of their problem of social decay?" Lopez Obrador said. His statement came amid proposals by some Republicans to use the US military to attack drug laboratories in Mexico.

A report by The Washington Post said fentanyl was also being hidden in passenger vehicles and trucks passing through official ports of entry in California and Arizona.

More than 90 percent of the fentanyl is coming through official border crossings, Customs and Border Patrol, or CBP, officers said. But data from a Senate report on port security released in 2020 show the CBP was scanning only about 15 percent of commercial cargo and 1 percent of vehicles.

Fentanyl is popular among drug cartels because it is more powerful than other narcotics, which allows for smaller quantities to be smuggled more easily than heroin or cocaine. Fentanyl is mixed with other drugs to make them stronger and more valuable, but it is so deadly that one person could carry sufficient quantities to kill an entire city.

Last year, the DEA seized more than 50 million fentanyl-laced pills and over 4,535 kilograms of fentanyl powder — enough to kill every US citizen.